1935–1942
United States Geological Survey, 1935
United States Geological Survey, 1936
United States Geological Survey, 1938
United States Geological Survey, 1937
United States Geological Survey, 1939
United States Geological Survey, 1610
United States Geological Survey, 1940
United States Geological Survey
![Untitled [portrait of Robert Gene Wilcox]](https://1.api.artsmia.org/25704.jpg)