PETEOGEAPHICAL MODIFICATIONS OF EHYOLITE. 393 



MICROSCOPICAL CHARACTERS OF THE EHYOLITE. 



From the description of the megascopical characters and the mode of 

 occurrence of the rhyolite just given, it is evident that the rock varies 

 greatly in the manner of its solidification and crystallization in restricted 

 areas, as well as throughout the whole region. Its microscopical characters, 

 therefore, may be expected to be equally varied. An examination of 315 

 thin sections of this rock shows that this is true within certain limits. There 

 is great variability among the sections, but it is confined to a definite range 

 of crystalline structures and to a limited number of minerals, and is so often 

 repeated in the rocks from different localities that a clear idea of the micro- 

 scopical characteristics of the rhyolite can be given only by treating them 

 systematically for the whole region. It then becomes a comparatively 

 simple undertaking-. 



The phenomena investigated by a microscopical study are expressions 

 of the mode of solidification of the lava and the extent or degree and the 

 manner of its crystallization. The mode of solidification of the rhyolitic 

 lava in this region is that of a surface flow of variable thickness. The lava 

 has been in part inflated and chilled into pumice, or has exploded and 

 formed tuff. Parts of it have solidified to dense glass, sometimes rendered 

 vesicular by large gas bubbles. But the greater mass of it has solidified in 

 the central parts of deep flows or streams, forming lithoidal rock, whose 

 crystalline character can not be recognized by the unaided eye. The 

 pumiceous portion occurs at the top of the flows, and passes downward into 

 dense glass, which in deep flows passes into lithoidal rock. The latter con- 

 stitutes the center of the flow, and generally passes again into glass at the 

 bottom, which may or may not pass into pumice, or more usually into tuff, 

 the pumice having been ground to pieces beneath the weight of the lava 

 stream. 



The extent or degree of crystallization in most cases manifests itself in 

 two ways: One is in the amount and kind of porphyritical minerals pres- 

 ent; the other is in the amount of microscopical crystallization that has 

 taken place in the rock, which has led to the production of microlites and 

 ao-greffations of two or more minerals in various kinds of arrangement. It 

 is evident in many instances that the first-mentioned group of products 

 resulted from a process of crystallization that antedated the arrival of the 

 lava at the surface of the earth, for the porphyritical crystals are scattered 



