In working out the past history of any given region, much of it is done on the basis of this series of equivalents. The finding of limestone, for instance, indicates that the given area was at one time under the sea to a considerable depth, that is from 100 to 1000 feet, but not ocean-bottom depths which run in tens of thousands of feet. Marble indicates the same thing, and so one can go on through all these types of rock. Gneiss Pl. 64 Gneiss is an old word used by the Saxon miners, and is often very loosely used. Here it is used in its structural sense, and a gneiss may be defined as: a banded metamorphic rock, derived either from a sedimentary or an igneous rock, and is composed of feldspar, quartz, and mica or hornblende, and is coarse enough, so that the constituent minerals can be determined by the eye. It corresponds to a granite, or some sedimentary rock like gravel or conglomerate. Due to the action of pressure, all the gneisses are banded, and the original constituent particles or crystals are distorted. The lines of banding may be long or short, straight, curved or contorted. When the banding is not conspicuous, the gneiss tends toward a granite. When the banding is thin and the structure appears flaky, the gneiss tends toward a schist. The color varies according to the constituent minerals, from nearly white, through red, gray, brown, or green to nearly black. Plate 64 shows one gneiss which is in a less advanced stage, the pebbles being simply flattened and the matrix partly altered to micaceous minerals, and a second gneiss which is so far advanced that the original constituents are all altered to other minerals and only the banded structure remains. This latter type would have required but little more heat to have completed the melting and changed this to a granite. Gneisses are very compact and have little or no pore space in them. They are hard and strong and resist weathering well, so that they are widely used as building stone: but they are not as good as granite for this purpose, as they split more readily in one direction and can not therefore be worked so uniformly as can granite. There are many varieties of gneiss, based either on their origin, composition, or their structure, as follows: Granite-gneiss is one derived by metamorphism from granite. Syenite-gneiss is one derived by metamorphism from syenite. Diorite-gneiss is one derived by metamorphism from diorite. Gabbro-gneiss is one derived by metamorphism from gabbro. Biotite-gneiss is one composed of quartz, feldspar and biotite. Muscovite-gneiss is one composed of quartz, feldspar and muscovite. Hornblende-gneiss is one composed of quartz, feldspar and hornblende. Banded-gneiss is one in which the banded structure shows clearly. Foliated-gneiss is one in which there is thin irregular layering. Augen-gneiss is one which has concretionary lumps scattered through it.