What is this gneiss, this granite, speaking of it with reference to its mineralogical character? It is a combination of silicates, with a base of alumina, potash, soda, and sometimes lime--_quartz_, _felspar_, and _mica_ form, by their simple aggregation, _granite_--it is thus a ternary combination, or composed of three minerals. _Quartz_, the most abundant of all minerals, is silica more or less pure and often crystallised. _Felspar_ is a crystalline or crystallised mineral, composed of _silicate_ of alumina, potash, soda, or lime; potash-felspar is called _orthoclase_, soda-felspar _albite_, lime-felspar _anorthite_. _Mica_ is a silicate of alumina and potash, containing magnesia and oxide of iron; it takes its name from the Latin _micare_, to shine or glitter. _Granite_ (from the Italian _grano_, being granular in its structure) is, then, a compound rock, formed of felspar, quartz, and mica, and the three constituent minerals are more or less crystalline. _Gneiss_ is a schistose variety of granite, and composed of the same minerals; the only difference between the two rocks (whatever may be their difference of origin) being that the constituent minerals, instead of being confusedly aggregated, as in granite, assume a foliated texture in gneiss. This foliated structure leads sometimes to gneiss being called _stratified granite_. “The term gneiss originated with the Freiberg miners, who from ancient times have used it to designate the rock in which their veins of silver-ore were found.”[33] [33] Cotta’s “Rocks Classified and Described,” by P. H. Lawrence, p. 232. The felspar, which enters into the composition of granite, is a mineral that is easily decomposed by water, either cold or boiling, or by the water of springs rich in carbonic acid. The chemical action of carbonic acid and water, and the action (at once chemical and mechanical) of the hot water in the primitive seas, powerfully modified the granitic rocks which lay beneath them. The warm rains which fell upon the mountain-peaks and granitic pinnacles, the torrents of rain which fell upon the slopes or in the valleys, dissolved the several alkaline silicates which constitute felspar and mica, and swept them away to form elsewhere strata of clay and sand; thus were the first modifications in the primitive rocks produced by the united action of air and water, and thus were the first sedimentary rocks deposited from the oceanic waters.