[1] After completing his Ph.D., Cross returned to the United States and began a career with the U.S. Geological Survey that would last until his retirement in 1925.

[1] From 1880 to 1888 he was based in Denver, Colorado, after which he moved to Washington, D.C.,[2] where he headed the Survey's petrology section.

[1] Besides his substantial field work, Cross was known for his pioneering research in normative mineralogy.

[3] Together with Joseph P. Iddings, Louis Valentine Pirsson, and Henry Stephens Washington, he devised a measurement of the chemical composition of rocks that came to be known as the CIPW norm, after their initials.

Published in a 1903 book, their method was more quantitative than previous approaches and introduced a novel nomenclature system.

While other petrologists generally judged their nomenclature to be too detailed and cumbersome, they embraced the quantitative approach of calculating a "norm" based on a rock's chemical composition, and the CIPW norm remains a staple of petrology.

[1][9][10] In 1925, his alma mater Amherst College awarded him an honorary Doctor of Sciences degree.