V.—On Parallel Structure in Rocks as indicating a Sedimentary Origin

Extract

Igneous rocks and crystalline schists are often associated with each other in such a manner as to compel the conclusion that either the igneous rocks have been formed out of the schists or the schists out of the igneous rocks. There are other cases in which the two kinds of rock are less intimately related; parallel structure is wanting, the junctions are sharply defined, and there is no evidence of an original mineral gradation. These are examples of the ordinary irregular intrusion of igneous rocks in schists. But there is a third group in which the characters are intermediate. In these rocks, structural parallelism is more or less distinct, and there is often a partial blending of the two kinds of rock at the line of contact; but other indications forbid the belief that the schists have been elaborated out of the associated igneous masses.