GEOGNOSY AND GEOLOGY. 97 
of this rock is but little known. What we do know is, that it consists of an 
intimate combination of feldspar with an unknown body, which, in many 
Cases, appears to be a zeolitic mineral; this union being sometimes so 
perfect as to give it the appearance of a simple mineral. Silex often 
communicates to it an extraordinary degree of hardness. A grey color is 
peculiar to it. It is often porphyritic, with crystalline glassy feldspar 
disseminated in its substance. The minerals generally included are meso- 
type, natrolite, chabazite, and apophyllite. 
Section 4. Obsidian. 
In rocks of this section the feldspar has been entirely fused, and only 
occasionally exhibits a crystalline structure. The action of fire is readily 
recognised, for which reason these rocks have more or less the appearance 
of glass. 
Pitchstone. This is of imperfect conchoidal fracture, of waxy lustre, and 
of grey, green, red, brown, and black color. 
Pearlstone is characterized by its granular or concentric lamination, and 
by its vitreous lustre, passing into iridescence. Its colors are grey, yellowish 
red, and brown. 
Obsidian occurs, like all the rocks of this section, in volcanic regions. It 
is characterized by its striking conchoidal fracture and perfect glassy lustre. 
The broken fragments have very sharp edges, and strikingly resemble a 
dark colored glass. Its black and brown colors depend on carbonaceous 
substances, for which reason obsidian yields a white result before the blow- 
pipe, by which it is distinguished from many slag-like rocks. 
Pumice, or Bimstein. This is nearly allied to obsidian, and differs only in 
its condition of aggregation. While the former is perfectly glassy, this has 
a more or less perfect glass-like cellular spongiform texture. It is found in all 
stages of the spongy scoriaceous character, running finally into obsidian. Its 
economical applications are well known. 
The rocks of this section frequently have their feldspar separated, and 
then occur as true porphyries. 
Order 4. Pyrozxene Rocks. 
The rocks of this order are characterized by pyroxene minerals, particu- 
larly malacolite, augite, diallage, and hypersthene. 
Section 1. Pyroxenes. 
Only one species occurs under this head, pyroxene rock proper. This is 
a granular foliated mixture of malacolite with augite, and is of an oil or olive 
green color. 
Section 2. Leucitophyre. 
This section includes rocks consisting of leucite and augite. The mixture 
occurs in various degrees of perfection. 
Leucitophyre (leucomelan). This exhibits a great number of modifica- 
tions depending on its structure. The crystalline granular form (also called 
leucite lava) contains leucite and augite in crystalline grains plainly distin- 
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