60 
AMERICAN GEOLOGY. 
PYROGENIC OR ERUPTIVE ROCKS. 
§ 48. General considerations respecting pyrogenic or eruptive 
rocks. Pyrogenic or eruptive rocks have five phases, each of 
which should be described. The five phases are recognized by 
the structure of their masses. The first have a symmetrical 
arrangement of their component parts in consequence of their 
crystallization. Thus the feldspar of granite is crystallized; 
and however small its particles, it is perfectly separated from 
the other minerals of the mass, whatever they may' be. The 
mica also is crystallized, and the quartz, though massive, is 
equally distinct. Each mineral composing the rock is clearly 
defined in its boundaries, and is a fact attested by the eye* 
This structure, which belongs to the mass and each mineral 
composing the mass, is the first phase among the pyrogenic 
rocks. The second phase preserves the isolation of particles, 
whose crystallization took place at the time when, and at the 
place where, they now remain; but the particles of minerals of 
the same in kind are arranged in parallel stripes or layers. 
The feldspar is arranged in its stripes or bands, the mica or 
hornblende in their bands respectively, and they may alternate 
with each other. The rock presents a striped aspect. Such 
an arrangement of parts is properly called lamination. Gneiss, 
mica and talcose slates, and hornblende are examples of this 
kind of structure. The third aspect, the separation of particles, 
is too indistinct to be recognized, or it is far less so than in 
granite. A single element of the rock may be imperfectly 
crystallized and isolated, while the particles of the mass remain 
indistinct, or it may be granular. Greenstones and porphyry 
are the most distinctive examples; the first is both massive and 
columnar, and it may be laminated, but the lamination is not 
distinguished by the arrangement of different minerals in 
parallel bands or stripes; but the laminae are all of the same 
kind of matter while separated from each in thin sheets. This 
structure is not uncommon. The fourth aspect, the rock is 
vesicular. In fusion the mass became pasty, and the confine- 
