418 DESCRIPTIONS OF ROCKS. 
phyry” is porphyritic diubase, or, since diabase cannot be 
distinguished mineralogically from doleryte, it 1s porphy- 
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 wary 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 
ip : 8. 

. Sa et 
a ts 
Basalt with the base unindividu- ‘‘Rhyolyte ;” 
alized. 
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 Rhyolyte (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 
kind of rock. 
2. ANHYDROUS AND Hyprous CRYSTALLINE Rooks.— 
Some eruptive rocks, like doleryte or trap, oceur 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 amygdaloidal—that is, contain little cavities that 
are often almond-shaped (the Latin amygdalum meaning 

