616 DESCRIPTIVE GEOLOGY. 



of mica can be detected macroscopically. Under the microscope, both mica, 

 brown hornblende, and small dark-green augite grains can be distinguished. 

 The gray, half-glassy gi-oundmass contains sphasrulites as large as a walnut, 

 which develop by decomposition a concentric-layer structure. The most 

 advanced stage of this decomposition closely resembles the larger litho- 

 physse, which would seem to sustain the opinion of J. Roth, that these 

 lithophysse are nothing but mechanically and chemically altered larger 

 sphserulites. 



About a mile from the edge of the cliffs is a little group of projecting 

 hills having a rude semicircular shape, suggesting the form of an ancient 

 crater, which are composed of a light pearl-colored rhyolite similar to that 

 found near the Warm Springs. Under the microscope, it is seen to be less 

 distinctly crystalline than that which forms the base of the cliffs, and to 

 contain- a large amount of tridymite. It also contains long prismatic bodies, 

 which are apparently altered biotite. 



At the base of the cliffs, at Stony Point, is a rhyolite which resembles 

 macroscopically the typical porphyritic rhyolite, already so often mentioned, 

 though this has rather a brownish-red color. It contains only crystals of 

 sanidin-feldspar, with some little plagioclase, in a compact felsitic ground- 

 mass. The monoclimc feldspars seem to be a good deal decomposed, and are 

 somewhat stained with iron oxide. Dark particles, apparently of specular 

 iron, can be distinguished macroscopically in the groundmass. Under the 

 microscopej this rock, with the exception of the absence of quartz, resembles 

 that found at the base of the cliffs further north. The groundmass is a 

 micro- crystalline aggregation of colorless particles, with grains of black 

 opacite and brownish ferrite. 



The basalt which forms the summit of the plateau at Stony Point is 

 a rather remarkable rock, having a light-brown color, through which are 

 mottlings of a light-green material. The rock is a remarkably fine-grained 

 variety, though somewhat vesicular, in which, by the unaided eye, no crys- 

 talline ingredients, with the exception of a few isolated feldspars and occa- 

 sional grains of quartz, can be distinguished. Under the microscope even, 

 the constituents are so fine as to be somewhat confused and difficult to dis- 

 tinguish. It is rather poor *in augite, and contains small reddish-brown 



