I believe that Robert Martin Lusk father was Samuel Igou Lusk b.1821 and his mother was Sarah Roberson b.1822
Refer to the transcription of census below.
Mike Lusk input: 1860 TN census, 4th Dist., Red Clay Post Office, Bradley Co., TN., page 87, June 22, 1860 shows; line 36, dwelling 564, family 564, Robert M. Lusk, age 1 or 9 (writing not clear), male, white, born TN. Living with his parents, father Samuel Igou Lusk, age 38, born TN.
Mike Lusk input: 1870 TN census, 3rd Civil Dist., Cleveland Post Office, Bradley Co., TN., page 19, July 13, 1870 shows; line 6, dwelling 129, family 129, Robert M. Lusk, age 18, male, white, attending school, born TN, has attended school within the year, can read and write. Living with his father, Samuel Igou Lusk, age 50, born TN.
Mike Lusk input: 1880 TX census, Corporation of Bonham, Precinct 1, Fannin Co., TX., page 1, Supervisors Dist. 2, Enumeration Dist. 22, June 1, 1880 shows; line 46, Main Street, dwelling 7, family 8, Robt M. Lusk (Robert M. Lusk), white, male, age 28, head, married, lawyer, born TN, father born TN, mother born VA. Living with him at time of census:
Clara Lusk, wife, age 25, born LA.
Oscar Lusk, son, age 3, born TX.
Susan Lusk, daughter, age 8 months, born TX.
Susan Pope, mother in law, age 51, born SC. (Clara's mother).
Mike Lusk, Calif.