On the Vertical and Meridional Lamination of the Primary Rocks

Abstract

The author described wide regions in several parts of the world as exhibiting in their geological structure the phænomena of successive vertical bands of schistose and crystalline rocks, parallel with each other, and having a meridional strike. This structural condition was illustrated by several extensive and highly finished sections, some of them traversing several hundred miles, made from the author's own observation in Panama, South America, Australia, and Ceylon. The section across the Andes, for instance, exhibited parallel bands of quartzites, porphyry, mica-schists, greenstone, granite, gneiss, hornblende schists, trachyte, crystalline limestone, talcose schists, and clay-slates, occurring in variable succession, with a N. and S. strike, and with an almost uniform vertical dip. In plains and other places where the laminated structure has not been disturbed by local causes, the cleavage planes were shown to be more or less vertical; but sometimes in high ridges with precipitous flanks the bands and laminæ of rocks drop on both sides, from want of lateral support, thus giving the appearance of a radial or fanshaped structure. Here and there on the edges of these laminated rocks rest horizontal sedimentary deposits; and it was pointed out that many of these exhibited at the point of contact with the older rocks evidence of their undergoing the process of vertical cleavage or lamination ; the lines of stratification becoming gradually obliterated. Even compact mud and soil lying on the edges of the schistose rocks have been observed by the author to be subject (under certain conditions) to cleavage and interlamination with calcareous and siliceous matter. Mr. E. Hopkins maintained that in all parts of the world the old crystalline or “primary” rocks exhibit (with local exceptions, insignificant when compared with the whole) a uniform vertical cleavage or foliation, with a north and south direction ; and that the rocks in those countries, like Australia, part of India, Siberia, South America, Central America, and California, which preserve this meridional uniformity, are productive of gold, platina, silver, and precious stones on the decomposed edges of the schists ; whilst those regions which have been disturbed or bent from their normal position are more or less productive in masses of the ordinary minerals, and are comparatively barren of the precious products. In speaking of the meridional structure, Mr. Hopkins alludes to the N.E. variation of the cleavageplanes in the northern hemisphere, more especially in the United States and Europe; but, nevertheless, he believes that the general uniformity approximates more nearly to the true meridian than the magnetic meridian does. The author observed also that, from his acquaintance during numerous mining operations with the deep-seated rock-masses of the Andes and elsewhere, he was convinced that the great base below was more or less granitic strongly saturated with mineral waters, and that it passed upwards by insensible gradations from a crystalline heterogeneous compound to a laminated rock (as gneiss), and still higher up to schists in vertical planes ; the peculiar varieties of the higher rocks being dependent on the mineral character of the “parent rock” below; the schistose rocks forming, in short, the external terminations of the great universal crystalline base. Mr. E. Hopkins referred to some important remarks on parallel lamination of nearly vertical rocks and on cleavage, made indeendently by Humboldt*, M'Culloch† Sedgwick‡, and De la Beche§ and, leaving for further consideration the question as to how the lamination and cleavage of rocks were brought about, he concluded by recommending the study of the primary rocks, with their various transitions and foliations, to the special notice of geologists, as being of extreme interest, and likely to throw great light on several important points in geological science.