Parr II. § iv.] MICROSCOPIC CHARACTERS OF ROCKS. 105 
- from thoroughly vitreous compounds like obsidian, on the one hand, to 
completely crystalline masses like some dolerites on the other. It 
occurs not only: in what are usually regarded as volcanie rocks, but also 
in plutonic. or deep-seated masses which there is reason to believe 
consolidated deep beneath the surface, as for instance in the Bode 
vein of the Harz and among quartz-porphyries associated with 
granites in Aberdeenshire. ‘The structure, therefore, cannot be 
regarded as certainly indicating that the rock in which it is found 
ever flowed out at the surface as lava. 
The final. stiffening of a vitreous mass into solid stone has resulted 
(ist) from mere solidification of the glass: this is well seen at the 
_edge of dykes and intrusive sheets of different basalt rocks, where the 
igneous mass, having been suddenly congealed along its line of contact 
with the surrounding rocks, remains there in the condition of glass, 
though only an inch further inward from the edge the vitreous magma 
has disappeared, as represented in Fig. 29 ; (2nd) from the devitrifica- 
tion of the glass by the abundant development of microfelsitic granules 
and filaments, as in quartz-porphyry, or of crystallites and crystals, 
as in such glassy rocks as obsidian and tachylite ; or (8rd) from the 
complete crystallization of the whole of the original glassy base, as 
may be observed in some dolerites and basalts. 
D. Cuastic.—Composed of detrital materials, such as have been 
already described (p. 102). Where these materials consist of grains of 

Fie. 13.—Ciastic Structure or InorGanic OrIGIN—SECTION OF A 
Piece or Greywacke. (10 Diameters. See p. 159.) 
quartz-sand, they withstand almost any subsequent change, and hence 
can be recognized even among the most highly metamorphosed series 
of rocks (p. 155). Quartzite from such a series can sometimes be 
scarcely distinguished under the microscope from unaltered quartzose 
sandstone. Where the detritus has resulted from the destruction of 
aluminous or magnesian silicates, it is more susceptible of alteration. 
Hence it can be traced in regions of local metamorphism becoming 
more and more crystalline, until the rocks formed of or containing it 
pass into true crystalline schists. 
