GEOGNOSY AND GEOLOGY. 89 
respect to geognosy ; and the interesting generalization has been made that 
all such belong to the class of the oxygenids (the class characterized by 
oxygen combinations). These bodies, so important in geognosy, are the 
following : 
1. Quartz, or crystallized silicic acid (silex), with the impure varieties, 
hornstone, silicious shale, jasper, and whetstone slate. 
2. Mica, or micaceous bodies, with the nearly allied chlorite, and talc. 
3. Feldspar, or. feldspathic minerals (among which we distinguish feldspar 
proper or orthoclase), labradorite, saussurite, albite, and oligoclase. 
4. Amphiboles, as hornblende. | 
5. Pyroxenes, as augite, diallage, and hypersthene. 
6. Calcareous, including the pure and impure formations, limestone, marl, 
tufa, &c. 
7. Dolomite, or carbonate of lime combined with carbonate of magnesia. 
8. Gypsum, hydrated sulphate of lime. 
9 Karstenite (anhydrite), anhydrous sulphate of lime. 
I. GENERAL PETROGRAPHY. 
The first part of geognosy or petrography teaches us the character of 
rocks or formations, and arranges them in systematic groups. Let us now 
turn our attention for a moment to this subject. Considering the rocks ina 
genetical point of view, they naturally fall under two heads; first, tsonomic, 
or those which were produced by a simultaneous crystallization or deposition 
from an aqueous or igneous liquid; and secondly, heteronomic, those which 
are composed of materials evidently formed at different times, or in different 
localities. These appear to have been brought together by subsequent 
agencies, and their parts stand entirely in accidental combination. 
All the parts of a rock, whether isonomic or heteronomic, must stand in 
actual combination. Considering the case of granite, which consists of a 
crystalline granular mixture of feldspar, quartz, and mica, we are enabled to 
assume that these three minerals, in actual combination, are the result of a 
simultaneous (not to take this term too literally) crystallization from a 
melted mass. Granite must therefore be counted among the isonomic 
formations. The same is the case with regard to syenite, the different 
porphyries, &c. The expression simultaneous cannot be taken literally. 
since in the gradual cooling of the liquid matter of the different minerals, all 
could not crystallize at the same instant of time, owing to the difference in 
their points of congelation: one must harden first, and the other occupy its 
interstices. By simultaneously, therefore. we understand a certain period, 
during which the force of crystallization, or the congelation and separation 
of amorphous masses, was continually acting. Limestone, which in many 
cases is amorphous, is also to be counted among the isonomic rocks, since 
its formation was also brought about by a simultaneous deposition of its 
particles. The case is different with the heteronomic rocks, in which we 
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