92 GEOGNOSY AND GEOLOGY. 
the quartz carries it into the former; a diminution of quartz, into the 
latter. The quartz is generally the common splintery kind, rarely waxy ; 
the feldspathic material is more compact, rarely sparrv. The predomi- 
nating color is grey of various shades. 
Section 3. Silicious Porphyry. 
Silicious porphyry, as the name indicates, is a silicious mass, in which 
feldspathic crystals are interspersed. The principal varieties are: 
Quartz Porphyry ; of rare occurrence, and only found in Sweden. The 
general color is white, as are also the crystals of feldspar. 
Hornstone Porphyry. This is often very similar to the preceding, but 
readily distinguishable by a simple blowpipe test. While quartz porphyry 
forms only a frit in the blowpipe flame, this melts, without much difficulty, 
into a white enamel. Its colors are grey, brown, yellowish; and in the 
‘mass thus constituted,crystals of feldspar are readily distinguishable. 
Silicious Porphyritic-slate. This beautiful rock is rather rare. In 
masses of jaspery, silicious slate, lie feldspathic crystals of a light color. 
This rock readily passes into hornstone. 
Jaspery Porphyry. This consists of feldspar crystals lying in a matrix 
of earthy jasper; and is of a lavender blue, grey, and greenish color. 
Order 2. Micaceous Rocks. 
The rocks of this order derive their name from containing mica, or 
chlorite and tale, which are closely allied to mica. Chlorite and tale may 
either replace or accompany mica. In the first series (the micas), mica and 
its allies occur pure and distinct. 
Section 1. Micas. 
Mica. It is principally the biaxial mica that occurs as a rock; the 
uni-axial being but rarely met with. It is a little remarkable, that where 
mica occurs in very large quantity, the lamin are never of very large 
size. It splits up into very thin plates, these consisting of minute scales of 
mica combined into layers. Its color is generally brown running into 
black, rarely silvery white. 
Chiorite. This is met with under different forms, namely, as chlorite 
slate, as chlorite rock, and as potstone. | 
Chlorite slate is the well known schistose chlorite of the mineralogist 
Its colors are seldom lively, passing from dark-green into greenish-grey. 
Chlorite rock is represented by common chlorite, which is generally 
coarsely slaty, and readily passes into potstone, which is an intimate 
combination of chlorite and talc. This potstone is not unimportant in a 
technical point of view, serving not only as a material for the must 
important utensils of some nations, but also admitting of conversion into 
various shapes by the art of the turner. 
Talc. The talcose slate of mineralogists generally exhibits a green, 
yellow, or white color. It passes, on the one hand, into chlorite and talc: 
on the other, into steatite. 
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