摘要：

Branney and Kokelaar (1992) emphasized that many features of pyroclastic flow deposits largely record the last processes involved in their formation, so Wolff and Turbeville's glib reminder of this philosophy at the end of their comment is quite unnecessary. Many structures in high-grade ignimbrites record non-particulate flow that occurred some considerable time after particulate aggradation ceased. This is not in question, and we pointed this out in our paper. It is the earlier deposition and deformation history of rheomorphic ignimbrites that is at issue. This is the aspect that bears more widely on the nature of pyroclastic flows and related eruptive phenomena. In another paper (Branney and Kokelaar 1994) we have documented evidence for hotstate remobilization and deformation of some stationary rheomorphic ignimbrites by post-emplacement disturbance (caldera collapse). However, in Branney and Kokelaar (1992) we provided evidence that shows that for most rheomorphic ignimbrites it is inappropriate to assume a twofold flow history (of hot remobilization after an initial flow came to rest). Progressive aggradation and agglutination provide the most straightforward explanation for many of the markedly irregular vertical variations in welding intensity characteristic of high-grade ignimbrites, just as progressive aggradation best accounts for vertical variations in sedimentary lithofacies and/or in chemical composition in other ignimbrites.

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