A discussion of <<The Rio Caliente Ignimbrite: Analysis of a Compound Intraplinian Ignimbrite from a Major Late Quaternary Mexican Eruption> by J.V. Wright G.A. MAHOOD Department of Geology, Stanford University, Stanford, California, 94305 USA WRIGHT (1981) introduces a new term, compound ignimbrite, and expands on the concept of an intraplinian ignimbrite, a term also used by WALKER et al. (1981) in a related paper on the Sierra La Prima- vera. A caldera-forming rhyolitic ignim- brite of moderate volume that erupted from the Sierra La Primavera 95,000 years ago serves as a ~(type >~ example of these two terms. Wright refers to this ignimbrite as the ~Rio Caliente Ignlm- brite >>, but it has previously been called the Tala Tuff (MAHOOD, 1977, 1980b, 1981), a name which has priority and has been adopted by the Mexican geologists working in the area. Introduction of the term ~compound ignimbrite)> is poten- tially confusing and largely unnecessary. More important, the concept is based on a shaky factual foundation if the Primavera example is the source of inspiration because two key characteristics of Wright's ~ compound ignimbrite ~ - a high-aspect ratio and rapid thinning as traced away from source due to loss of individual flow units - have not been demonstrated. As a result, volumetric eruptive rates inferred from the ~compound~ nature of the ignimbrite may be suspect. In the follow- ing paragraphs I will show that the appar- ent high aspect ratio and rapid thinning are a product of Wright's having compared stratigraphic sections on opposite sides of the caldera boundary, which separates outflow sheets from intracaldera tuff at least four times as thick. Neither has the ignimbrite's ~ intraplinlan>~ nature been documented; in fact, the available field Bull: Volcanol., Vol. 46-2, 1983 evidence suggests that it is not intraplin- ian (i.e., it is not ~interbedded between co-eruptive plinian fall deposits~0. The term ~compound ignimbrite>~ is potentially confusing because of the well- established use of SMITH'S (1960) ~ compound cooling unit>>, which embod- ies a very different concept. It also seems an unnecessary addition to the already overburdened lexicon applied to pyroclastic deposits. One need only state that the ignimbrite in question consists of one, four, or numerous flow units. If one wishes to characterize an ignimbrite by the ratio of its thickness to runout, one can refer to a ~ low-aspect ratio >~ or ~ high- aspect ratio ignimbrite ~, as did WAIA~R et al. (1980). Wright acknowledges that uplift subse- quent to caldera collapse led to dissection that produced the present exposure of intracaldera ignimbrite and overlying caldera-lake sediments in the Sierra La Primavera. It follows that the thickness of near-source ignimbrite exposed in Wright's type section at Rfo Caliente is controlled by structural disruptions during caldera collapse and subsequent uplift. Thus the significance of the ignimbrite's aspect ratio as a measure of eruption dynamics is questionable. In any case, because the base of the near-source ignlm- brite is not exposed, the calculated aspect ratio is a minimum value. In order to attribute a genetic signifi- cance to the thickness of near-source ignimbrite, Wright had to argue against the possibility of significant intracaldera