94 _  GEOGNOSY AND GEOLOGY. 
Calcareous Clay Slate, an intimate admixture of carbonate of lime and 
clay slate. It is a valuable rock, furnishing excellent materials for soils. 
While common clay slate does not act very favorably on vegetation, this 
variety permits the finest forest growth. ~It is of a dark color, for which 
reason it is put to a very peculiar use in some countries. This consists in 
sprinkling it, when crumbled into pieces, on snow-covered ground, thus 
accelerating the melting of the snow, at the same time that an useful manure 
is added to the soil. 
Section 3. Horn Slate (Hornschiefer). 
This section contains only one rock, horn slate. It is a tough solid 
substance, and is often a great hindrance in mining. Hornblende often | 
enters into the more usual combination of mica and quartz, and 
communicates to the usual grey or dusky black color, a tinge of green. 
Section 4. Flagstone (Gestellstein). 
These are rocks consisting of a crystalline schistose mixture of micaceous 
minerals with quartz. First to be mentioned is: 
Flagstone. In the mixture just referred to, mica is the prevailing 
component; chlorite and tale occur more rarely. As any one of these 
three mineral substances occurs combined with quartz, we have micaceous, 
chloritic, and talcose flagstone. The lamination of flagstone is very distinct. 
The quartz granules are generally invisible, being concealed by the mica. 
These quartz grains often occur in lumps, and form entire beds by their 
combinations; the micaceous matter investing these lumps produces a 
knotty undulating lamination. The color of the rock depends on that 
of the mica. The chlorite flagstones are generally green, the talcose, 
white. 
Hornblende-, graphite-, marble-, dolomite-flagstone, schorl- and micaceous- 
iron slate, arerocks belonging in this place: they are distinguished from 
each other by the proportion in which one or other of the above-mentioned 
ingredients enters into its composition; this taking place frequently in 
such quantity, that the micaceous substance is entirely displaced. 
Section 5. Gneiss. 
This section is composed of rocks which consist of micaceous substances, 
quartz, and feldspar, and possess a decidedly crystalline laminated structure. 
The first species to be mentioned, is 
Common Gneiss. The mica is arranged in parallel layers, imparting to 
the rock its schistose structure ; color grey, brown, and black: the quartz 
is generally grey, and in no great proportion, and is even sometimes entirely 
wanting. The colors of the feldspar are mostly grey and white, the red is 
rare. Similar modifications are found in gneiss to those in flagstone, 
effected by the micaceous element. Thus we have a chlorite, or a tale 
gneiss, as chlorite, or talc, replaces the mica. On account of the solidity 
of gneiss, it is much used for building and other similar purposes. 
Hornblende is sometimes added to the other constituents of gneiss, thus 
producing hornblende-gneiss. The micaceous element is sometimes entirely 
displaced by the hornblende. 
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