DACITES. 39 



beginning of the dacite-rhyolite eruptions was signalized by the appearance of the 

 Heller dacite, which formed numerous small cones along lines of weakness and was 

 poured forth in relatively limited quantities. 



Microscopic character*. Under the microscope the Heller dacite shows a 

 brown glass groundmass, which is sometimes spherulitic and which contains 

 numerous porphyritic crystals, nearly always broken, of quartz, feldspar, and 

 biotite. It resembles the Brougher dacite. Striated and unstriated feldspars are 

 about equally represented. The latter are probably in large part orthoclase, 

 while in one slide examined striated feldspars proved to be andesine. 



FRACTION DACITE BRECCIA. 



Location. A considerable part of the southern half of the area mapped is 

 covered with a soft brownish or greenish rock of volcanic origin. This rock is 

 sometimes solid, is occasionally dimly horizontally layered or packed, is at times 

 definitely stratified, and even contains well-bedded tuffs. The material is dacitic, 

 essentially like the Heller and the Brougher dacite. It does not occur in the 

 relatively elevated northwestern half of the area mapped but in the southeastern 

 half it spreads far beyond the map limits and occupies large portions of the low 

 areas between the hills. 



Thickness. This formation varies in volume, but is frequently several hundred 

 feet thick. Perhaps the greatest thickness actually demonstrated is at the New 

 York Tonopah shaft, which is 745 feet deep and is entirely in this formation, 

 except for intrusive bodies of the Tonopah rhyolite-dacite or included fragments 

 of earlier rocks. 



Conditions of eruption. In places the dacite belonging to this formation is 

 nonfragmental and of the nature of a flow. But it is invariably soft and friable. 

 It grades into a common type where it is often difficult to decide whether or 

 not the rocks are of fragmental character. They often consist of broken, close- 

 packed, medium-sized fragments of more or less pumiceous dacite, but under the 

 microscope show no signs of fragmental origin. An explanation of their origin 

 that accounts for their different features is that these rocks were partly or 

 entirely volcanic mud flows, in which the highly pumiceous and aqueous lava 

 was mingled with such an excess of heated waters that it was partly broken and 

 ground up in the course of the flowing. Rock of this nature grades with no 

 sharp line into thick, unstratitied accumulations of brownish or greenish pumice 

 fragments, which are of considerable size, and which grade into similar masses 

 of smaller pieces. In some parts of such deposits a rude stratification or layer- 

 ing may be observed, and occasionally there are thin layers of well-stratified tuff 

 (fig. 1). These pumice accumulations point to explosive eruptions. In them are 



