General names.
The difficulty of distinguishing many of the foregoing rocks from each other by any means available in the field, owing to the minuteness of the crystals, and to the gradation of one type of rock into another, makes it desirable to employ certain general names which will correctly express the leading character of the rock without implying a knowledge of the precise mineral composition. A convenient term of this kind is greenstone, which merely indicates that the ferromagnesian minerals are prominent and usually give a greenish or dark cast to the rock. The greenstones embrace the diorites, dolerites, some of the gabbros and the basalts, and may even extend to the peridotites and some of the more hornblendic of the granitoid rocks. Another convenient name is trap, which may be used for any dark, heavy igneous rock. The name (from trappe, stairs) refers to the step-like arrangement which the edges of the superimposed sheets of lava often take, especially when the lava is of the free-flowing, basaltic kind.
Fig. 347.—Brecciated limestone, Calciferous formation. One mile south of Highgate Falls, Vt.
The term basalt is sometimes used to embrace any of the very fine-grained dark igneous rocks. In such cases, it covers the very fine-grained dolerites, diorites, peridotites, etc. The term granite was used originally for any coarse-grained crystalline rock, and there is a tendency to revive this early use. In general descriptions, some of our best petrographers call any coarsely crystalline rock (e.g., coarse-grained syenites, diorites, gabbros, etc.) granite. The term granitoids may be used with strict propriety to cover all rocks of this class.
DERIVATION OF SECONDARY ROCKS.
Rocks, though commonly made the symbol of the abiding, are subject to constant slow changes. Through these changes newer rocks have been derived from older ones, and still others in turn from these derivatives, and so on in an endless chain. All derived rocks are conveniently termed secondary, though they may be several generations removed from the primitive rocks, and even the primitive rocks, as we now understand them, may be themselves derived. The ordinary changes of rock are most active at or near the surface, and the processes of such change have already been discussed in part under the titles “Weathering” (pp. [54] and [110]), “Erosion” (pp. [119–123] and [342–349]), “Transportation” (pp. [115–119] and [354–355]), and “Deposition” (pp. [177–204] and [355–363]).
Fig. 348.—Quartzitic breccia. About one-third natural size. (Photo. by Church.)
