The problems of the tectonics of the Coast Ranges have interested the writer for a number of years. His attention was first called to this field of research by the discovery that certain high-angled faults in the vicinity of Mount Diablo, middle California, were older than the Pliocene-Pleistocene folds of that area. The mapping showed conclusively that several of these faults originated in pre-Miocene time. Also, it was found that some of them had formed the margins of the former basin of deposition, and that, in the case of marine invasions into the area, the shorelines were in close proximity to the faults. Detailed mapping on both sides of these lines of faulting showed sudden thickening and thinning of the sediments; on one side would be found overlaps and erosion contacts together with marked hiatuses in the sequence, and on the other side would be a much greater thickness . . .