326 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



of Fleuriau de Bellevue and Dolomieu, to pulverise the 

 apparently homogeneous rock-varieties, to separate the particles 

 by weight, and test them partly below the microscope, partly 

 with the magnet, partly by chemical means ; but this manner 

 of research proved far from successful, as it was extremely 

 difficult to identify the minute mineral particles. It showed, 

 however, that basalt was a composite rock. 



The Scottish geologist, Professor William Nicol, in 1827 

 introduced a method of preparing thin sections of fossil 

 woods to be examined by the microscope, and about the same 

 time constructed a polarising microscope for the special 

 investigation of crystals. The insight of this gifted man in 

 petrographical pursuits, no less than in respect of the difficult 

 problems of the geology of the Scottish Highlands, failed to 

 carry conviction into the minds of his contemporaries. A 

 few petrographers certainly adopted his method of examining 

 fossil woods, and it was by this means that Goppert was 

 enabled to detect the important constituents of coal. 



In the hands of Ehrenberg, the microscope proved of epoch- 

 making significance. By its use Ehrenberg made the dis- 

 covery that a number of widely distributed rocks, soft in 

 character, such as chalk and tripolite, as well as certain lime- 

 stones from the older formations, were entirely composed of 

 the skeletons of lowly organisms (diatoms, foraminifera). 

 Ehrenberg's work on chalk and chalk-marls was published 

 at Berlin in 1839; fifteen years later, in his Mikrogeologie, he 

 gave a complete account of his microscopic investigations on 

 the composition of sedimentary deposits, the work being 

 enriched by a very large number of excellent illustrations. 



Although Ehrenberg's method of microscopic examination 

 of friable and earthy rock-material had been so eminently 

 successful, it did not seem as if it could be adapted for the 

 investigation of the harder rocks. The thin splinters of a 

 crystalline rock were not sufficiently transparent even when 

 imbedded in Canada balsam, and NicoFs optical method of 

 identifying the mineral fragments was little known. Besides 

 Nicol himself, David Brewster and Humphrey Davy interested 

 themselves in the microscopical examination of the structural 

 relations of minerals, and the frequent fluid inclusions of rock 

 minerals. Scheerer in 1845 identified the hemicrystalline 

 structure of many apparently homogeneous rocks, and in 

 transparent chips of crystals examined by transmitted light 



