138 



A. F. BUDDINGTON 



indicates that the replacing mineral is pyrophyllite, it has been 

 calculated as that alone. Owing also to the liability of error 

 involved in assigning the elements of the pinitic rocks to the correct 

 minerals in the right proportions, only a rough estimate of their 

 mineral composition is given. The iron oxides and magnesia of 

 No. 7 must be present as an integral part of the white mica molecule, 

 as no other mineral except quartz can be distinguished in thin 

 section. 



TABLE II 

 Recalculated Analyses 



PETROGRAPHY 



No. i (228 Z) i c). This specimen was taken near the top of a 

 50-foot banded reddish-gray felsite flow. In thin section the tex- 

 ture varies from microfelsitic to very minutely microcrystalline and 

 the flow lines are marked by hematite dust. The flow lines are 

 sharply curved and crenulated and several are replaced by quartz, 

 especially in the loops of the curves, so that the rock analyzed 

 as representing the composition of the original rhyolite only approxi- 

 mates such an unaltered condition. 



No. 2 (228 F 2). This rock is a drab to fawn-colored micro- 

 spherulitic rhyolite with secondary iron oxide in veinlets and specks. 

 In thin section the groundmass is a finely microcrystalline aggregate 

 of quartz and orthoclase, with fan-shaped microspherulitic areas. 

 Secondary quartz is present as grains and lenses, as well as replacing 

 portions of the spherulitic aggregates. A minute amount of sericite 

 and quartz occurs along fractures. 



