﻿Cvi PEOCEEDINGS Or THE GEOLOGICAL SOCIETY. [May I908, 



pages on this subject, in his little Yolume of ' Researches in 

 Theoretical Geology,' published in 1834. But his efforts also 

 failed to arouse general recognition of the true value of a branch 

 of geology which had ceased to be actively cultivated in this 

 country. In contrast to this neglect, the study of minerals and 

 rocks, under the prolonged impulse that had been given by Werner, 

 was pursued in Germany with persistent energy, and produced many 

 papers and treatises. 



But even on the Continent further progress in the investigation 

 of this branch of geology was seriously retarded by the difficulty 

 in determining the actual mineral composition of the large series 

 of rocks which have too close a texture to allow their individual 

 mineral constituents to be recognized, either with the naked eye or 

 with a lens. Extensive use had, indeed, been made of chemical 

 analysis, although it then lacked the delicate refinement to which 

 it has since been carried. The general chemical composition of 

 rocks had been fairly well ascertained. Nevertheless, although it 

 might often be possible to infer with some confidence from chemical 

 analysis what separate minerals have been aggregated together in 

 the formation of a particular rock, no means had yet been discovered 

 of actually determining the presence and proportions of the mineral 

 ingredients in fine-grained masses. 



In the early years of last century, indeed, Cordier had described 

 a method of crushing down such rocks, washing and separating 

 the grains of their powder, and examining these grains under 

 the microscope.^ By this process it was possible to ascertain the 

 mineral constitution of some rocks, and to study the characters of 

 their individual minerals. But, as these minerals were detached 

 from each other and from their matrix, and were often opaque^ 

 comparatively little light could be obtained on the intimate structure 

 of the rocks composed of them. It may, therefore, be affirmed that, 

 up to the middle of the nineteenth century, no means had been 

 devised for determining with precise accuracy the various micro- 

 scopic elements of rocks, the order in which these elements solidified, 

 and the conditions in which their solidification took place. 



But from that time onward a great revolution was accomplished 

 in the methods of rock-analysis. Petrography was so completely 

 transformed as to be ushered, with widened capabilities and freshened 

 interest, into a new career of rapid development. Erom being a 



^ Ann. de Chimie & Physique, vol. iii (1830) p. 285, Cordier's paper was 

 read to the Academy of Sciences in 1815. 



