214 



STRUCTURAL GEOLOGY. 



Diorite and diabase have already been described. It is 

 only necessary to say that, when occurring intrusive, 

 they are finer-grained than the massive varieties. Felsite 

 is a fine-grained, light-grayish rock, consisting essentially 

 of orthoclase and quartz. Porphyry is a rock consisting 

 of fine-grained feldspathic paste, with disseminated large 

 crystals of feldspar (Fig. 123). But any rock is said to be 

 porphyritic if it consists of fine-grained paste with large 

 crystals of any kind disseminated. 



II. VOLCANICS, OR TRUE EEUPTIVES. 



The rocks of this group are distinguished from those 

 of the other, both by texture and mode of occurrence. By 

 texture they are not only finer-grained (micro-crystal- 

 line), but there is always more or less of uncrystalline or 

 glassy base or cement, showing that the fused mass has 

 cooled too quickly to allow complete crystallization. Often, 

 also, as already explained under volcanoes (page 139), 



these rocks are in 

 a wholly glassy and 

 even in a scoriaceous 

 and tuf aceous condi- 

 tion. The principal 

 rocks of this group 

 are given in the ac- 

 companying table. 



Trachyte may be 

 taken as a type of 

 the acidics. It is a light-colored rock, with a rough feel 

 (hence the name), consisting essentially of orthoclase with 

 more or less quartz. When the quartz-grains are con- 

 spicuous, it becomes rhyolite. PJionolite is a dense vari- 

 ety, of light-grayish color, which splits into slabs in 

 weathering, and rings under the hammer almost like 

 metal (hence the name). Obsidian and pumice are glassy 

 and scoriaceous varieties of trachyte. 



