On the Structure of the Crystalline Rocks of the Andes, and their Cleavage-Planes

Abstract

I t is a well-known fact to those who have traversed large areas of uncovered crystalline rocks, that they present a structure more or less vertical. In North America, Scotland, and Sweden the diversified edges of the crystalline series may he traced for many miles, presenting a north-easterly structural hearing. Where argillaceous sedimentary beds have been deposited on crystalline rocks, having the above vertical structure strongly developed, I have frequently observed the latter, by a slow chemical action, in the course of time obliterate the seams of deposition in the former, and cleave the beds in conformity to the angular position of the primary structure and the disposition of the crystals in the crystalline base. These cleavage-planes pass through mountains of undulated beds, and preserve throughout the various contorted seams, the geometrical parallelism of the structure of the parent rock below; thus plainly showing the effect of a polar crystallizing force, acting gradually and insensibly on the whole mass in one direction to an indefinite extent. Many thousand square miles of this vertical structure may be seen in South America, and in magnificent sections several thousand feet deep, exhibiting a great variety of colours, crystallization, and transitions of the primary series; as may be observed on reference to the sections exhibited to the Society, which have been carefully made from a very. laborious, survey carried, on during, many years, geologically, mineralogically, barometrically, and trigonometrically, this survey was not alone confined to the surface, but it comprehends the results of very extensive