418 



DESCRIPTIONS OF ROCKS. 



pliyry " is porpliyritic diabase, or, since diabase cannot be 

 distinguished mineralogically from doleryte, it is porpliy- 

 ritic doleryte ; and, in these and other like cases, the being 

 porphyritic is a characteristic of minor value. 



Sometimes igneous rocks exhibit under the microscope a 

 fluidal texture; that is, the material, when examined in 

 sections, shows wavy lines or bands, which are evidence of 

 a former fluid state, and of movement or flowing when in 

 that state. One variety of this texture is represented in fig- 

 ure 8 (from Zirkel), giving a magnified view of an eruptive 





Basalt with the base unindividu- 

 alized. 



Rliyolyte ;" Fluidal texture. 



rock from the head of Louis Valley, Nevada ; and another 

 in figure 5, p. 416. Such rocks have been comprised under 

 the general name of Rliyolyte (from the Greek for flowing) ; 

 but this fluidal texture is presented by rocks of different 

 mineral constitution, and is hence not a proper basis for a 

 hind of rock. 



2. Anhydrous and Hydrous Crystalline Rocks. — 

 Some eruptive rocks, like doleryte or trap, occur both an- 

 hydrous and hydrous. The latter, unlike the former, have 

 the constituent minerals clouded in aspect, however thinly 

 sliced, and often changed in part to a green chlorite — a hy- 

 drous mineral — and also sometimes to other hydrous species. 

 Such rocks, moreover, have less lustre, and very frequently 

 they are am ygdaloidal— that is, contain little cavities that 

 are often almond-shaped (the Latin amygdalum meaning 



