During the first 30 years following Gilbert's explanation, offered in 1874, for the mountains of the Basin and Range province as dissected fault blocks, the evidence of their uplift on faults was physiographic in the sense of being dependent on facts of surface form; namely, a simple base line indifferent to the structure of a range-block along at least one of its sides. The evidence was all the better if post-faulting erosion had advanced so little as not to have altogether destroyed those modified remnants of the fault face seen in the spur-end facets by wearing them too far back from the initial fault plane; it was all the worse if post-faulting erosion had advanced so far as not only to consume the spur-end facets, but also to wear the mountain face well back from the fault trace and thereby give it an irregular base line. Physiographic evidence of faulting would then be lost and the origin of a range so much eroded would remain, as far as such evidence goes, uncertain. ; © 1932 American Association for the Advancement of Science.