Thomas Stephen Julian was b. 1814 Lanivet, Cornwall, England and died 1851 Grass Valley, Nevada, CA.
His wife was Christian Roberts b. 17 Nov 1822, England.
Their child was Stephen Thomas Julian b. 25 Oct 1849 in Warren, Jo Daviess, Illinois. He married, had a family and died 22 Dec. 1923at 1817 S. Main, Rockford, Winnebago, Illinois.
Christian Roberts Julian married Elijah Deweitt Covill 9 Nov 1855 in Jo Daviess Co., Illinois.
A Stephen Thomas Julian B. 25 Oct 1849, Mineral Point, Wisconsin Father: Thomas S. Julian b. England Mother: Christain Robert, b. England Residence: Rockford, Winnebago, Illinois Occupation: City Fire Chief (retired) Spouse: Mary Hart Died: 22 Dec 1923 @ 1817 S. Main, Rockford Burial Place: Greenwood Source: Illinois Deaths & Stillbirths 1916-1947, familysearch.org/
Elijah Dewitt Covill b. 7 Oct 1832 in New York. Christain died 28 Feb. 1921 in Rockford. They had several children and he died 4 Feb 1926 in Rockland. Source: Family tree cited above.
Of course you don't want to enter the information from the family tree until you verify it.
Thomas Julian's death occurred one year after California became a state. I will be difficult to find a record.
First I'd visit Nevada County Lookups, cagenweb.com/nevada/nclookups.html . On the second and third pages you'll find a lot of books listed. Send an email to the volunteer and ask them to look it up in those books.