More ONeil Cousins

We have another set of potential ONeil cousins.

Two people who are second cousins to each other share DNA with my father and his three siblings.

The common ancestors of these DNA matches are Andrew O'Neil, born about 1798 in Ireland and his wife, Bridget Goodfellow, born about 1798, possibly in County Cavan, Ireland. They settled in New York City in the late 1840s.

1855 New York State census, New York City
Household of Andrew ONeil and wife, Bridget.

In the 1855 New York State census, Andrew and Bridget resided in New York City with their five children, ranging in age from 12 through 22: Michael, Peter, Andrew, Margaret, and Patrick. Note that Michael O'Neil had been in the city for 11 years, while the rest of the family was present for six.

To work up these DNA connections, you need to find descendants of this couple in the DNA database. But- even if this couple has descendants in the DNA database, the connection is distant, so no detectable and reportable shared DNA may exist.

ONeil is a popular Irish surname with a variety of spellings. Over half a million people lived in New York City in 1855. (Today about 8.5 million people reside in Manhattan.) Tracing the marriages, children, grandchildren, and so forth of ONeils is tricky.

Margaret ONeil married Bartholomew Neville. When she died in 1907, she was widowed and childless. She left behind a will that was probated in Kings County, New York. From this will, we find out that she had another brother, Terence. He was not with the family in the 1855 census. More about Terence later.

Excerpt of will of Margaret Neville (born ONeil) (1842-1907).
Proved November 19, 1907 in Kings County, New York.

"Second. I have not mentioned my
brother Terence O'Neil's name in this Will
nor made any provision for him herein, for
the reason that I consider him well enough
provided for with his own property and poss-
essions."

Andrew O'Neil died in 1868. He was originally buried in Jersey City at Saint Peter's Cemetery in Section G South, plot 14. In 1885, Andrew, other O'Neils, and Murphys were removed to Calvary Cemetery in Queens, New York. (The Newark Archdiocese maintains an electronic database of burials at Find A Loved One Search.)

A brief notice of the death of matriarch Bridget appeared in the local paper The Sun in 1885. She was described as a "native of county Cavan, Ireland."

Obituary of Bridget O'Neil from the newspaper The Sun in New York City, July 4, 1885.
Note: July 4, 1885 was a Saturday. The obituary states she died on Saturday, July 3.

"O'NEIL. - On Saturday [Friday], July 3, Bridget O'Neil, aged
84 years, native of county Cavan, Ireland.
Funeral to take place from her late residence, 87 Washington st., on Monday, July 6."

Maybe Bridget's husband, Andrew ONeil, was also from Cavan. Some records survive for County Cavan in the 1800s. I looked for Charles ONeil and Catherine Brougham, the supposed parents of my third great grandmother, Mary Neil. I found a marriage record for Patrick Neal and Cath Brogan from 1824 in Urney Parish. Could this have been Mary's parents? More research is needed. Maybe the name is more commonly spelled Brogan and not Brougham.

Marriage of Patrick Neal and Cath Brogan in Urney, County Cavan.
August 18, 1824.
Witnesses Patrick Brady and Petra??? Brogan.
Collection at Ancestry

Summation

My line:
Mary Neil (later O'Neill), born about 1830 in New Jersey, later Ireland.
Daughter of Charles O'Neill and Catherine Brougham of Ireland.

Potential relatives number 1:
Hugh ONeil, born about 1834 in Ireland.
His wife, Margaret ONeil, born about 1838 in Ireland. Daughter of James O'Neil and Margaret.
Settled in Washington.

Potential relatives number 2:
Andrew O'Neil, born about 1798 in Ireland. His wife, Bridget Goodfellow, born about 1798, possibly in County Cavan, Ireland. Settled in New York City.

More on Terence ONeil

To trace Terence ONeil, we cannot stop at the records available at Ancestry. Additional documents were filed to probate Margaret's estate. Some are available at FamilySearch.

Probate Administrator Records for Kings County, New York.
Available on FamilySearch.org Image Group Number 004362601.

"Administrator Records" provide several pages listing names and addresses of Margaret's relatives. Terence ONeil, her brother, lived at 153 Steuben Street in Jersey City, New Jersey.

Below is Terence's death certificate from 1910 and a picture of his gravestone in Holy Name Cemetery in Jersey City.

Death certificate of Terrance O'Neill, died June 16, 1910
in Jersey City, Hudson County, New Jersey.
Father- Andrew. Mother Bridget.

Holy Name Cemetery, Jersey City, Hudson County, New Jersey
O'Neill gravestone in Block M
March 30, 2019

This DNA connection was only recently brought to my attention in the current year, 2026. Yet the dates on the records of Terence is from 2019.

Why did I gather this information seven years ago? Because Terence's son, Andrew Leo ONeill (1863-1941), married into my mother's family. My mother is not an ONeil descendant, but she has cousins who are.

Terence's third great granddaughter is among my DNA matches. She and I are third cousins, once removed on my mother's side. We share a segment of DNA. Ancestry does not report shared DNA between her and my father.

Note that Ancestry does not provide you a list of everyone who shares DNA with you. Both testing parties would need to upload their DNA files to GedMatch to check for shared DNA. Not finding someone listed as a DNA match at Ancestry does not mean that you don't share DNA.

ONeil DNA Cousins

Have relatives of Mary Neil (1830-1898) been identified among the DNA matches at Ancestry?

Mary Neil, or Neill, or ONeill, was my great great great grandmother. She was born around 1830. On the record of her first marriage in 1847 to my third great grandfather, Calvin Cook (1826-1889), names of parents were not recorded. The names of her parents are from the record of her second marriage to Nicholas Keating (1825-1898) in 1892: Charles ONeill and Catherina Brougham. Mary's death certificate from 1898 only provides her mother's first name, Katie.

Only the names of Mary Neil's parents are known.

In contrast, Calvin Cook's ancestors are well-plotted at this point in my research. As a bonus, this was an intermarrying group with lots of descendants. As a result, I have hundreds of DNA matches who trace back their ancestry to the ancestors of Calvin Cook: Wiggins, Peer, Post, White, Vanderhoof, Stegers, Young, Hopler, DeMouth, Muller, etc.

Ancestors of Calvin Cook, mostly from Morris County, New Jersey.
They intermarried. Their descendants share more DNA because of this intermarrying.

DNA Strategy

This is the strategy for identifying relatives of Mary Neil amongst the DNA matches:

Mary Neil and Calvin Cook have around fifteen descendants in the DNA database at Ancestry. Use of the "Shared Matches" feature (only available for an additional monthly subscription) finds other relatives. These shared matches might be connected through Calvin Cook OR Mary Neil. If the shared match has ancestry amongst Calvin's ancestors, then the connection is probably not through Mary Neil.

A few people do not share matches with the Calvin Cook Morris County ancestry. These are potentially related through Mary Neil's ancestors.

One of the problems is that we are working with tiny segments of DNA that trace back at least six generations. If we land in Ireland, there are few written records to demonstrate a relation.

DNA Matches of Interest

We will look at three people who match descendants of Mary Neil and Calvin Cook, but do not match anyone in the Morris County group. There are more matches who fit this scenario, but they have cryptic usernames, no trees, private trees, scant trees, and do not answer messages.

All three matches share less than 20cM of DNA with my group, or about one small segment of DNA. This indicates that the relation is likely third cousin or more distant.

The sorting feature places the DNA match's matches at the top. (Also a paid feature.) This is such a helpful mechanism for figuring out how relations relate to one another.

We will label the three matches as Person 1, Person 2, and Person 3. Person 2 and Person 3 are siblings. Person 1 turned out to be their second cousin.

Their common ancestors are Hugh ONeil (1835-1888) and Margaret ONeil (1838-1906) of Ireland and Washington, United States. (Washington became a state in 1889.) Person 1 descends from their son George. Person 2 and Person 3 descend from their son Charles.

Household of Hugh ONeil and his wife, Margaret, in the 1880 census.
Washington Territory, United States of America.
Children: Evelena, Minnie, John, Charles, Francis, George, and Kate.

Having the same surname as Mary Neil is great, but this is a common Irish surname and means little until we have paper documentation. Spoiler alert: we do not have any records connecting them at this juncture.

Hugh ONeil was a veteran of the Civil War. He died in 1888, the year before Washington became a state. His death record was a line in a ledger book. His parents were not listed.

Margaret ONeil died in Oregon in 1906 while visiting her children. Her death certificate provides the names of her parents as James ONeil and Margarete, last name unknown by informant, G W ONeil.

Death certificate of Margaret O'Neil.
Died March 15, 1906 in LaGrande, Union County, Oregon.
Collection online at Ancestry

Was Margaret also ONeil? The records of her children provide no other surname. Some of their records are easily viewable online, such as marriage, death, and Social Security Death Index. No surname other than ONeil or ONeill was given for their mother.

Yet her memorial page at Find A Grave provides Cook as her birth name. Hmmm.

Find A Grave memorial page for Margaret ONeil (1838-1906)
Buried in the Mountain View Cemetery in Walla Walla, Washington.
How was her birth name of Cook found?

Cook is a common name. My Cook ancestry is Dutch, not Irish. The name was Van Der Koeck.

Future Research

More DNA connections will appear with new leads.

More records will find their way in an easily accessible format online.

I'd like to see Brougham turn up amongst the matches.

Tangled Family Lines at First Presbyterian Church Cemetery, Rockaway, New Jersey

On May 9, 2026 I visited the First Presbyterian Church Cemetery in Rockaway, Morris County, New Jersey. It was a rainy day, so the stones were darkened. I prefer overcast, dry days for cemetery visits, but this is how the schedule worked out.

This is an active church and cemetery. This is their website.

I saw some Wiggins stones. More articles on Wiggins are coming.

Jody at grave of Daniel G Wiggins (1827-1895) and wife Phebe Belcher (1830-1888).
First Presbyterian Church Cemetery in Rockaway, Morris County, New Jersey.
May 9, 2026.

Daniel Gard Wiggins (1827-1895) was a granduncle of Sarah Lavinia Merritt (1842-1909), wife of Private Charles Youmans Cook (1833-1913). Charles was the reason for this visit.

Elizabeth Minton (1822-1901) and husband Jonathan Wiggins (1820-1877).
First Presbyterian Church Cemetery in Rockaway, Morris County, New Jersey.
May 9, 2026.

Jody at the Cook/Merritt graves.
First Presbyterian Church Cemetery in Rockaway, Morris County, New Jersey.
May 9, 2026.

Here's the stone of Catherine Shaw (1825-1891). She was the first wife of Nicholas Keating (1825-1898). After Catherine's death, Nicholas remarried to Mary Neil (1830-1898), my third great-grandmother. Finding this marriage lead to discovering the names of her parents. 

Jody at the grave of Catherine Shaw (1825-1891),
wife of Nicholas Keating.
First Presbyterian Church Cemetery in Rockaway, Morris County, New Jersey.
May 9, 2026.

Tangled lines: Catherine Shaw was a granddaughter of Conrad Hopler (1730-1816), making her a first cousin, once removed of Mary Neil's first husband, my third great-grandfather, Calvin Cook (1826-1889).

Mystery DNA Cousin Demystified

A few years ago at 23andMe a close match appeared for my father, his siblings, and their third cousin, Bob, on their shared Cook/Neil line of Morris County, New Jersey.  Common ancestors are Calvin Cook (1826-1889) and Mary Neil (1830-1898).

The amount of shared DNA ranged from 1.66% with my uncle to 3.82% with cousin Bob.

The probable relation would be second to third cousin.  The variance in amount of shared DNA is within normal.  Or the higher amount could indicate that this mystery cousin is closer to Bob.

Either way, the person ignored my requests to connect through the 23andMe website.

Recently, 23andMe required users to not be anonymous.  This person bypassed this non-anonymous requirement and instead blocked sharing requests.

This person won't make or break my family tree, so I moved on.

Then Cousin Bob's cousin contacted me.  They share ancestors Patrick Bernard Brady (1830-18xx) and Elizabeth Duffy (1837-1918) of County Meath, Ireland.  They were the parents of Mary Brady (1870-1942), wife of Francis Asbury Cook (1851-1919).

She wondered how Bob was so closely related to her highest DNA match.  This Mystery DNA Cousin had limited contact with Bob's cousin and revealed Brady ancestors on two separate ancestral lines from two counties in Ireland, Cavan and Donegal.

In this situation, we are not using haplogroups to assign relationships or ancestral lines.
We are using them to confirm that we are dealing with the same elusive DNA tester.

The shared percentage with Bob, as well as the haplogroups, were the same for the Mystery Brady Cousin as for the Mystery Cook/Neil Cousin.

So if the Mystery Cousin is the same for both situations, this explains why Cousin Bob shares more DNA with the Mystery Cousin.  They are related through Cook/Neil ancestors as well as the separate Brady line.  For Bob, these lines merged in his great grandparents, Francis Cook and Mary Brady.

The above diagram is my theory on how the Mystery Cook/Neil Cousin (Mystery Brady Cousin) is related to my branch.  If this person comes forward, we can revise the this diagram if needed.

Genealogy Brick Wall Crumbled

Brick wall down!  The parents of Mary Neil, my 3rd great grandmother, have been discovered.

Five generations of my father's family tree.
The focus of this discussion is Mary Neil (1830-1898), a great great grandmother of my father.

Mary Neil married Calvin Cook in 1847.  Like most of the marriages recorded in Morris County, New Jersey in this time period, the names of parents were not included.

New Jersey County Marriage collection at FamilySearch.org.  Free index and free images.

In other records, Mary was born in New Jersey around the year 1830.  As time went on, some of her children recorded her name as O'Neill on their documents and gave her birthplace as Ireland.

In the 1870s, the family relocated to Jersey City in Hudson County.  Calvin died in Jersey City in 1889.  I found his death certificate with no difficulty in the Archives in Trenton; however, I could not locate his final resting place "at Dover" until someone kindly posted on FindAGrave the stone for Calvin and Mary's son, William Cook, who died in 1871.  The family plot is located at Locust Hill Cemetery in Dover, Morris County.  This was discovered over two years ago.

Mary's final passage is recorded on the same stone as her husband, Calvin.  She died August 9, 1898.

Great!  I had Mary's date of death.  But I still could not find a death certificate or obituary for her.

Time passed.  Recently I was at the Morristown and Morris Township Library and checked the Morris County newspapers again.  The few papers for the area for 1898 were published weekly, so I did not have too much to sift through.  This time, an obituary in the paper "The Iron Era" from Dover caught my attention.  Mary A Keating died on August 9, 1898 and was buried at Locust Hill Cemetery-- just like my Mary.  She was the wife of Nicholas Keating and lived in Rockaway.  No other family members were mentioned.

Could Mary Keating be Mary Neil, widow of Calvin Cook?

Digitized newspaper collection at the Morristown and Morris Township Library.  Free on-site usage.

At home, I checked the online index of New Jersey marriages at FamilySearch.  Calvin Cook died in 1889, so if Mary remarried, the date would be in the 1890s.  And there it was.  Mary remarried in 1892 (this is the correct year) in Jersey City to Nicholas Keating.

FamilySearch.org.  New Jersey Marriages.
This is a free index.  The images are not online.  They are at the Archives in Trenton.

This marriage record was my best chance of finding out the names of Mary Neil's parents, as the record was created during Mary's life.

I looked through the index at FamilySearch for Mary's death certificate, but found no matching entry.  Both Mary and Nicholas died in 1898 and their estates were probated through the Surrogate's Office of Morris County.

MorrisSurrogate.com
This is a free service to search probated estates in Morris County, New Jersey.
The actual files are in the court house.  [The other twenty New Jersey counties are online at FamilySearch.org.]

At the Archives in Trenton, I found Mary's marriage record to Nicholas Keating.  Her parents were listed as Charles O'Neill and Catharine Brougham, both of Ireland.  (This explains the source of the names for two of Mary's children.)

Witnesses were Margaret Tower, Mary's daughter; and Harry Tower, Mary's son-in-law.

I found a death certificate for Mary Keating.  No day of death is on the certificate; only the month and year- August 1898.  Maybe this is why it missed the index?  I looked at the microfilm roll of deaths from 1 July 1898 through 30 June 1899, Morris County, surname K.  This same method did not produce a death certificate for Nicholas Keating, who died 21 December 1898 according to his estate papers.

Whoever provided the information for Mary's death certificate only knew that her mother was "Katie."  This is why it is best to try to obtain a record created during the person's life.  (Cause of death was "cerebral haemorrage," probably a stroke.)

I went through the census to find Nicholas Keating.  In the 1860 federal census in Rockaway, Nicholas and his first wife, Catherine Shaw (1825-1891), were living next door to Mary Neil and her first husband, Calvin Cook.  32 years later, both of their spouses would be dead and Nicholas and Mary would marry each other.  For the record, Catherine Shaw was not merely a neighbor.  She was a first cousin of Calvin's father; the common ancestors were Conrad Hopler (1730-1815) and Elizabeth DeMouth (1735-1812).

In the 1895 New Jersey state census, Mary and Nicholas were residing together in Rockaway.  Had I not made the connection with the obituary, this piece of the puzzle could have provided a big clue.  I do not know how the three people named Nix tie into this yet, but look at the last person in the household, a child, Francis A Peck.  He is a grandson of Mary.  His parents were Calvin Peck (1848-1923) and Catherine Cook (1854-1885).  Francis was born on the 28th of April in 1885 in Jersey City.  Three weeks later, on the 17th of May, his mother, Catherine, died.

Mary's estate papers clearly list her surviving children and her grandson.  But estates are organized by the surname of the deceased, not by those who inherit.

Mary also left money to Louisa Lee "of Dover, N[ew] J[ersey], niece of my deceased husband Calvin Cook."  I don't know why Louisa received this special treatment.  Louisa's mother was Anna Cook, a sister of Calvin; her father was Jesse Lee.

Next I need to research Charles O'Neill and Catharine Brougham.  I'm not convinced that they were Irish.

Secondary Sources

I finally tracked down the death certificate of Charles Cook, my great great grandfather.  He was a resident of Newark, Essex County, New Jersey when he died December 6, 1937.  I looked at the death certificates year after year until I found the correct one.  This is sometimes what has to be done if the date cannot be obtained from another record.

Unlike the death certificate of his father, Charles' death record provided the name of the cemetery where he was buried, Locust Hill in Dover, Morris County, New Jersey.  His father's record provided only the town of burial.  I made several trips, inquiries, and searches until I found the relevant Cook grave in Dover.

Cook family plot at Locust Hill Cemetery in Dover, Morris County, New Jersey.
Charles Cook (1859-1937) does not have a marker.

When you look at a record, it is important to keep in mind which pieces of information are primary versus secondary.  The date of death and location are primary, as the facts were recorded at the time of the event.  The date and place of birth and names of spouse and parents are secondary sources.

Charles' date of birth is quite specific on his death record:  October 22, 1857.  Unlike most people born in the 1850s, Charles' birth in Denville, Morris County, New Jersey was recorded at the state level.  Charles was born June 11, 1859.  The birth record is a primary source for Charles' birthdate.

The 1860 federal census gives Charles Cook's age as 1 year.  This is consistent with a birth in 1859.  Ages skew in later census years.

The names of Charles Cook's parents are not consistent with other records.  His father was Calvin Cook (1826-1889), not William.  His mother was Mary Neil (1830-1898) or O'Neil, not Caroline.  Charles' wife was Minnie Caroline Bishop (1866-1910).

As you gather documents, evaluate if you are looking at primary or secondary sources.  Remember, though, that primary is not synonymous with absolute truth.

DNA studies: Sketching genetic branches

Family Tree Maker 2012

In the family tree above, orange represents the inheritance path shared by my father and his paternal first cousin once removed, D. W.  The purple represents the inheritance path shared by my father and his maternal third cousin, R. S. They are related one more generation back to Calvin Cook and Mary Neil.

At this time, my father and these two cousins have tested their DNA at 23andMe.  The key to figuring out your matches is to test family members from different branches of your tree.

My father matches both R. S. and D. W. on the same area of Chromosome 13.  You have two sides to each of your autosomes-22 of your 23 chromosomes.  [The 23rd pair is the sex chromosomes and they are inherited differently.]  One side is from your mother, the other side from your father.  Current DNA analysis that we are using for genealogy is not able to report which side of the chromosome is being read.  Instead, the matches for both sides are mixed together, making it very hard to distinguish your paternal matches from your maternal matches.  My father has Colonial American ancestry on both his paternal and maternal sides, making it difficult to decide which of his parent's trees we should look to for the common ancestor.

23andMe DNA comparison
My father versus his third cousin and his first cousin once removed.
The area of focus for this discussion is Chromosome 13.

Now that I have both cousins matching in the same area, I can look for matches in the database who match my father on this same area of Chromosome 13 and compare them against the known cousins.

Using R. S. as a guiding point, as he shares the largest segment with my father in this area of Chromosome 13, we find that J. G. and P. J. J. also match my father on this area of Chromosome 13.  Note that where one chromosome breaks, the other starts.  I find this quite often in the matches.

We compare these matches to R. S.  This will represent our test to see if they match on my father's mother's side, specifically the Cook/Neil branch.

P. J. J. and J. G. do not match my father's maternal third cousin.  This is not conclusive proof that they do not match, as errors occur in testing and slight mutations occur in every generation.

We have to try the other option, that P. J. J. and J. G. match through my father's father, Clifford Lutter.  So we look at the ancestry of my paternal grandfather.  His mother, Laura Winterton, was of Colonial American extraction.  His father, Howard Lutter, was of entirely German descent.

Distant genetic cousin J. G. was born in the United States and has heavy German ancestry.  One of his ancestral names looks a lot like one of my ancestral names in this branch of my tree.  We are unable to find a common ancestor in our trees at this time, but I would like to place J. G. in the Lutter/Uhl branch.

P. J. J. has very limited access to her ancestral tree, more of a blank slate.  I actually find unknown ancestries easier to work with because they have no preconceived notions of who and where their ancestors were.  Based on the break in the DNA segment, my hypothesis was that P. J. J. matched through my father's paternal grandmother, or the Winterton/Dunn branch.  When I tested my cousin D. W. of the Winterton/Dunn branch, my hypothesis was that he would match P. J. J. but not J. G.

Here's the comparison.

I was right, but with a catch.  P. J. J. does indeed match D. W., but not on our area of focus, Chromosome 13.  They match on Chromosome 10.  I would feel more certain attributing P. J. J. to the Winterton/Dunn branch if the match were on Chromosome 13.  This may be the best we can get right now, so I will proceed at this point in our studies that P. J. J. matches in the Winterton/Dunn branch and J. G. matches in the Lutter/Uhl branch.

If I can identify more genetic matches in either branch, we can establish more common matches and be more certain that we are assembling the genetic trees accurately.

If any of my fellow 23andMe users have any advice on utilizing both close cousins, please let me know.  Thank you.

Locating the Graves of Calvin Cook and Mary Neil (Finally)

After several years, I have located the final resting spot of my great great great grandparents, Calvin Cook and Mary Neil.  Here is the story.

Calvin Cook and Mary Neil were both born around 1830 in Morris County, New Jersey.  They were married in the 1840s (different records exist- we'll save that for another post).  I know little about Mary Neil's parentage, but the Cooks were a Morris County fixture for generations before Calvin's arrival. 

I combed through area cemeteries and burial grounds, including the burial place of Calvin's parents, Stephen Cook and Elizabeth Vanderhoof, at the aptly named Cook Cemetery, now engulfed by the Rockaway River Country Club.

Locating a large cemetery plot is a great way to find out names and dates of deaths, making it easier to locate their death certificates.  Depending on which is easier and more accessible to you, you can aim to locate the death certificate or the cemetery first.  I actually had Calvin Cook's death certificate and obituary.  He died in 1889 and burial was "at Dover."  Where in Dover?  That was my issue.

Location of burial of Calvin Cook, died 1889, on his death certificate.
New Jersey Department of Health

Two official cemeteries exist in Dover, Morris County, New Jersey:  Locust Hill and Orchard Street.  I visited both.  Orchard Street Cemetery has no office on site.  I walked around, but saw no Cook graves.  Records for Locust Hill do not start until the late 1890s, so there was no record of Calvin Cook and no matching record for Mary.

The Dover Historical Society has a compilation of gravestone transcriptions for Orchard Street Cemetery.  No Cooks are listed.

Gravestone transcriptions by Brianne Kelly-Bly.
Obtained online through the Dover Historical Society website.

Next I looked for Calvin Cook and Mary at the burial locations of their children.  I was making my way through the children when I stumbled on the Hudson County, New Jersey probate records for Calvin Cook.  Calvin had died in Jersey City, where he was living.  Mary, his widow, submitted papers from Hudson County to Morris County enabling her to dispose of real property held in Morris County.  The actual will and certifications were held in Hudson County.  In Calvin's will, all of the expected children were mentioned, except William Cook.  I thought I had William in the census through 1930.  I erased this 20th century death and looked for a William Cook who died before Calvin in 1889.  This was the key.

Children Francis, James, Charles, Margaret, and Catharine are mentioned, but not William.
1889 Probate papers of Calvin Cook
Hudson County, New Jersey
Obtained online at FamilySearch.org

Someone posted a gravestone for William H Cook, son of Calvin and Mary, 1849-1871.  This fit the names and time frame.  And the location?  Locust Hill Cemetery in Dover.

So back to Locust Hill Cemetery I went, hoping that someone would recognize this stone:  white with a fallen cross.  I also hoped that Calvin and Mary were buried nearby.  The caretaker did recognize the stone- he had lovingly re-cemented the cross many times over the years.

The taller dark stone nearby was for William's parents, Calvin Cook and Mary!  Finally I had found them.

Mary died in 1898.  This is puzzling because I have already searched for a death certificate for her through the early 1900s.  I must return to the Archives at Trenton with this death date in mind.

Calvin and Mary's daughter, Catherine Cook, wife of Calvin Peck,
is also buried in this plot.  Like the other burials, her death date predates
the cemetery's records.