GEOGNOSY AND GEOLOGY. 99 
Order 3. Feldspathic Rocks. 
Feldspathic minerals form the constituents of these rocks. Common 
feldspar itself, or orthoclase, most generally constitutes this ingredient ; in 
rarer cases, the vitreous feldspar or sunadin, as also the compact feldspar. 
These feldspathic substances may be replaced by their allies, albite and 
oligoclase. Quartz and mica are generally united with feldspar, sometimes 
so intimately that the mixture can scarcely be distinguished from a chemical 
combination. 
Section 1. Granite. 
Granite, that well known and important rock, is a mixture of feldspathic 
and micaceous minerals with quartz. It is one of the most interesting 
rocks, both on account of the great number of its modifications, and the 
wide extent to which it prevails. Its color is very variable, depending 
upon whether the predominant feldspathic matter be flesh-color, greyish- 
white, or greenish; the quartz transparent, milk-white, grey, rose-red, or 
sapphire ; the micaceous matter brown or silver white; or whether the 
mica be replaced by chlorite or tale. The two latter minerals often occur 
combined, forming what is distinguished by many geologists as protogine. 
Granite has the same general composition as gneiss, feldspar predominating © 
in the former, mica in the latter. The principal difference lies in their 
different modes of occurrence; and genetically considered, the two rocks 
have an entirely different formation. Gneiss has always a decided 
lamination or stratification, which is rarely seen in granite, and then in an 
entirely different manner. The grain of granite is present under all its 
modifications ; it may be large and coarse, or small and finely granular ; 
and while in the latter the different minerals can no longer be distinguished 
by the naked eye, in the former each constituent particle appears to 
occupy an almost independent position. In the coarse-grained granite, the 
mica, for instance, is sometimes found in plates which are more than a foot 
square. Granite is especially favorable to cultivation: decomposing 
readily, it furnishes many inorganic matters of vast importance. Feldspar, 
in whatever combination it may be met with, readily decomposes under the 
influence of water, heat, &c., and after single soluble constituents have been 
removed, it then forms kaolin or porcelain clay. Granite is a very 
valuable building material, and is of importance in the manufacture of 
porcelain and glass. 
Syenite. This may be considered as a granite, in which hornblende 
occupies the place of mica. For this reason it is sometimes called 
hornblende-granite. Mount Sinai, a name associated with all the traditions 
of our faith, is composed of this rock; on which account the nam? 
sinaite has not inaptly been suggested for it. Like granite, it is often found 
porphyritic. If the hornblende, or its homologue, disappear, we have a 
rock called granitelle. The occurrence of mica in this, appears to be a 
superfluous, not necessary constituent. 
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