[2] He published many studies on the volcanoes of California, but is most noted for his "The Geology of Crater Lake National Park" in which he recognized the nature of the collapse of the crater and extended the work to develop the principles of volcanic caldera formation.

He did extensive early work on the geology of Central America (often sketch-mapping from the windows of second-class buses), and of the Galapagos Islands.

In Latin America, Williams put to good use his early background in archeology.

For instance, he used petrographic techniques to trace the origin of stone used in the giant Olmec sculptures of La Venta, Tabasco Mexico.

His drawings of the microscopic features of rocks of all types were used exclusively in the very successful textbook, Petrography, by Williams, Turner, and Gilbert.