The Structure and Origin of Stratified Rocks 1

Abstract

IN his address last year the author treated exclusively of the structure and origin of limestones, and now confined his remarks to the structure and origin of all other stratified rocks. In the first place he considered the question of the origin in crystalline rocks of the material, and described those peculiarities in external form and internal structure, which would enable us to determine the true nature and origin of the grains of sand and other materials met with in stratified rocks. He next considered the formation of the very fine-grained particles met with in clays and mud, as derived from the mechanical wearing down of minerals like quartz, which cannot be decomposed, or from the chemical decomposition of others like felspar and hornblende. The materials thus formed mechanically and chemically by the complete weathering of crystalline rocks are to a great extent in a state of equilibrium, and not prone to undergo further change, whereas the minerals in volcanic ash are to a considerable extent in a state of such unstable equilibrium that they soon undergo further important changes. A deposit of this nature might thus soon be more altered than one of the other type in vast geological periods. Amongst other facts of the like kind it may be named that the large amount of very fine-grained micaceous mud deposits found in some of our earlier strata was shown to be in all probability derived from certain quartz felsites, in which the base is to a large extent composed of very minute crystals of mica.