James Gilluly has rejected the interpretation of Wrucke and Silberman (U.S. Geol. Survey Prof. Paper 876, 1975) that a thrust fault and tear fault mapped by Gilluly and Gates (U.S. Geol. Survey Prof. Paper 465, 1965) as structures bounding the upper plate of the Roberts Mountains thrust at Mount Lewis are parts of a ring fracture around an area that underwent volcanic collapse. In his discussion (this volume) of our paper, Gilluly fails to consider important questions that we presented in support of the subsidence hypothesis. Instead of answering these critical questions, Gilluly merely recapitulates the interpretations that he and Gates gave in Professional Paper 465. We presented new information, including a map of one critical area along the cauldron boundary where, among other significant differences in geologic interpretation, we found the ring fault where previously no steep fault was shown. Gilluly believes that the paucity of dikes along the ring fracture at Mount Lewis is highly anomalous for cauldrons. However, the amount of dike rock is comparable to that in known cauldrons (some cauldrons have none) and is what might be expected at high levels in subsidence structures that have undergone relatively little resurgent igneous activity after collapse. Gilluly concludes that in formulating our interpretation of volcanic collapse, we have ignored much evidence that he and Gates have presented on thrust faulting; in the Shoshone Range. On the contrary, we have considered their ideas and have reinterpreted them using new evidence that strongly supports the concept of cauldron subsidence at Mount Lewis.
