Gneiss.—This is the most important of all rocks. It forms not far from one-half of New England, and a very large proportion of the earth’s crust. The name (pronounced same as _nice_) is known to have originated among the Saxon miners, but its precise derivation is lost in obscurity. To find out what this very important rock is, we will consult specimen 41. The first glance shows us that it is not, like the rocks we have just been studying, composed of a single mineral, but of several minerals, the most conspicuous of which is the pink feldspar—orthoclase. This we recognize as a feldspar: (1) by its hardness, which is a little less than that of quartz, and distinguishes it from calcite, a mineral having the general appearance of feldspar; (2) by its color, which separates it from hornblende and augite; and (3) by its cleavage, which distinguishes it easily from quartz. Finally, we know it is orthoclase, and not plagioclase, by its general aspect, and by its association with an abundance of quartz, which is the next most important constituent of the rock. The quartz is less abundant than the orthoclase, and more easily overlooked, yet anyone familiar with the mineral will not fail to recognize it. It forms small, irregular, glassy grains, entirely devoid of cleavage, and scratching glass easily. On weathered surfaces of the rock the orthoclase becomes soft and chalky, while the quartz remains clear and hard, and then the two minerals are very easily distinguished. Besides these, there are numerous black, thin, glistening scales, which we can easily prove to be elastic, and recognize as mica. In most books on the subject, these three minerals—orthoclase, quartz, and mica—are set down as the normal or essential constituents of gneiss. But it is now recognized by the best lithologists that we may have true gneiss without any mica; or we may have hornblende in the place of mica. Quartz and orthoclase are the only essential constituents of gneiss; and when these alone are present, we have the variety known as binary gneiss. The addition to these essential constituents of mica, gives micaceous gneiss; and of hornblende, hornblendic gneiss. Of these three principal varieties, the micaceous gneiss is by far the most common and important. The mica may be either the white species, muscovite, or the black species, biotite; but it is usually the former. Orthoclase is the predominant constituent in all typical gneiss, usually forming at least one-half of the rock. The orthoclase may, however, be replaced to a greater or less extent by albite, or even by oligoclase. But we frequently see the term _gneiss_ carelessly, or ignorantly, applied to rocks which are destitute of feldspar, though having the general aspect of gneiss. Augite rarely occurs in gneiss; and hence, when we observe a gneiss containing a black mineral which we know is either augite or hornblende, it is pretty safe to call it the latter.