In 1853 he published a Geological Map of the United States, and the British Provinces of North America.

He subsequently made a geological section extending from the Mississippi River to the Pacific Ocean.

His Lettres sur les roches du Jura et leur distribution géographique dans les deux hémisphères (published 1857...1860) included one of the earliest proposals that a land bridge had once existed between the Old World and New World.

In 1861 again returned to the United States, when he assisted Louis Agassiz in initiating the Museum of Comparative Zoology,[2] and was in charge of its palaeontological division from 1860 to 1864.

Subsequently, he devoted himself to scientific research until 1875, when he again began service for the United States government,[3] and accompanied the Wheeler Survey to Southern California.