I am guessing that Mary McEwen Morgan was the daughter of Robert McEwen, who was born about 1821 in Mississippi, and that this Robert McEwen was the son of Archibald McEwen, who born about 1790 in North Carolina. Archibald McEwen, who was born 1790 in North Carolina, also had a son named Archibald McEwen, who was born about 1842 in Mississippi. In others words, I am guessing that Mary McEwen Morgan had both a grandfather and an uncle named Archibald McEwen, but that her father was Robert McEwen. My guess is based on information found in census records, and I have pasted them below. You will certainly need more evidence or documentation to confirm your lineage. You must be very careful when linking one generation to the other. Information found on census records must always be corroborated with a variety of other records, and with as many as you can find. Census records are a good place to start.
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I am assuming on this census that Archibald McEwen, born 1790 in N.C., has a son named Robert, born about 1821, and a son named Archie, born about 1842. Relationships are not given on the 1860 census.
1850 census of Pike County
Archibald McEwin 60--born North Carolina about 1790
Mary McEwin 48
Thomas P McEwin 30
Robert McEwin 28--born Mississippi about 1822
Daniel McEwin 26
Abraham S McEwin 24
John McEwin 22
James McEwin 20
Josephus McEwin 16
Benton McEwin 12
Martha McEwin 10
Archibald McEwin 8--born Mississippi about 1842
Silas McEwin 7
Samantha McEwin 5
Source Citation: Year: 1850; Census Place: Police District 3, Pike, Mississippi; Roll M432_380; Page: 31B; Image: 67.
Source Information:
Ancestry.com. 1850 United States Federal Census [database on-line]. Provo, UT, USA: Ancestry.com Operations, Inc., 2009. Images reproduced by FamilySearch.
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1860 census
Relationships are not given on the 1860 census.
Mary McEwin 58
Benton McEwin 23
James McEwin 31
Source Citation: Year: 1860; Census Place: , Pike, Mississippi; Roll M653_589; Page: 454; Image: 460; Family History Library Film: 803589.
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1860 census
Relationships are not given on the 1860 census.
Robt McEwin 40--born Mississippi
Anna Pucket 73--born North Carolina
Source Citation: Year: 1860; Census Place: , Pike, Mississippi; Roll M653_589; Page: 447; Image: 453; Family History Library Film: 803589.
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1870 census
Relationships are not given on the 1870 census.
Mary McEwin 69
Archey McEwin 24--born Mississippi about 1846
Martha McEwin 22
Source Citation: Year: 1870; Census Place: Township 4 Range 9, Pike, Mississippi; Roll M593_745; Page: 139B; Image: 284; Family History Library Film: 552244.
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1880 census
Archy McEwen 38--born in Mississippi about 1842--father born in North Carolina
M.E. McEwen 33--wife
Joe C. McEwen 14
Josephene McEwen 12
Caldonia McEwen 8
A.B. McEwen 3
Source Citation: Year: 1880; Census Place: Beat 3, Pike, Mississippi; Roll T9_662; Family History Film: 1254662; Page: 409.3000; Enumeration District: 39; Image: 0219.
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There is an error in the census index. Arcada Brewer is shown on the original census living in the household of Robert McEwen, but in the index she is shown in a separate household.
1880 census
Robert McEwen 60--Single--born Mississippi--father born North Carolina
Brewer Arcada 30---(Unclear whether she is a widow)--(Relationship was not given)--born Mississippi--cooking and washing
Brewer Emily 5--Daughter--born Louisiana
Blum Matilda 11--Niece--born Louisiana
Source Citation: Year: 1880; Census Place: Beat 3, Pike, Mississippi; Roll T9_662; Family History Film: 1254662; Page: 409.3000; Enumeration District: 39; Image: 0219.
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1900 census
Archie Mc Ewen 57--born October 1843 in Mississippi--father born North Carolina
Martha Mc Ewen 46--wife
Hettie Mc Ewen 20
Gussie Mc Ewen 17
Jones Mc Ewen 15
A Mc Ewen 13
Source Citation: Year: 1900; Census Place: Beat 3, Pike, Mississippi; Roll T623_825; Page: 11B; Enumeration District: 109.
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I am assuming that this is your grandmother, still unmarried in 1900. She is listed as the daughter of Robert McEwen, who was born about February 1821 in Mississippi.
1900 census
Robt Mc Ewen 79--born February 1821 in Mississippi--father born in North Carolina
Mary Mc Ewen 18--daughter--born January 1882 in Mississippi
Julia A Mc Ewen 14--daughter
Source Citation: Year: 1900; Census Place: Beat 3, Pike, Mississippi; Roll T623_825; Page: 11B; Enumeration District: 109.
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I am assuming that this is your grandfather, still unmarried in 1900.
1900 census
Jefferson Morgan 50
Elizabeth Morgan 43
Dewitt Morgan 22--son--born January 1878 in Mississippi
Edward Morgan 13
Ida Morgan 10
Louanna Morgan 8
Luther Morgan 8/12
Source Citation: Year: 1900; Census Place: Beat 3, Pike, Mississippi; Roll T623_825; Page: 2B; Enumeration District: 108.
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Your grandparents.
1910 census
Dewitt Morgan 31
Mary Morgan 28--wife
James Morgan 6
Daisy Morgan 2
Source Citation: Year: 1910; Census Place: Beat 3, Pike, Mississippi; Roll T624_755; Page: 5B; Enumeration District: 100; Image: 560.
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1910 census
Archie McEwen 67 --born in Mississippi--father was born North Carolina
Martha McEwen 65
Hettie McEwen 30
Janes McEwen 25
A McEwen 22
Source Citation: Year: 1910; Census Place: Beat 3, Pike, Mississippi; Roll T624_755; Page: 13A; Enumeration District: 101; Image: 625.