MICROSCOPIC PETROLOGY. 767 



class, whether of the most compact, hard, and homogeneous 

 appearance, or occurring in the softest and finest powder, like the 

 ashes and dust frequently thrown out by volcanoes, a similar 

 crystallized arrangement and structure is present and common to 

 them all. Lavas, Trachytes, Dolerites, Diorites, Porphyrites, 

 Syenites, Granites, &c, all possess the same general structural 

 features, serving to distinguish the Eruptive rocks as a class from 

 all others." Again, it has been shown by Mr. Sorby that Micro- 

 scopic examination often allows the Minerals formed at the time of 

 the solidification of the rock, to be distinguished from such as are 

 the products of subsequent alteration by the action of water, or by 

 atmospheric or other agencies. In the case of Sedimentary Rocks, 

 it frequently happens that Microscopic examination affords the only 

 means by which the problem of their origin can be resolved ; the 

 most .compact and apparently homogeneous specimens being thus 

 shown to be aggregations of more or less rounded and water- worn 

 grains (often less than l-1000th of an inch in diameter) of Quartz, 

 weathered Felspar, Mica, soft and hard Clays, Clay-slate, Oxide 

 of Iron, Iron-pyrites, Carbonate of Lime, fragments of Fossil 

 Organisms, &c, arranged without any trace of decided structure 

 or crystallization. And in rocks exhibiting Slaty Cleavage, this 

 may often be clearly demonstrated to be the result of pressure 

 applied at right angles to the structure itself, thereby causing an 

 elongation or flattening-out of some, along with a sliding move- 

 ment of other of the particles. — The foregoing examples are 

 sufficient to indicate the value of Microscopic inquiry in that 

 department of Geology which includes the study of the composition 

 and origin of Rocks, and which is now known as Petrology. It is 

 a study, however, which can only be profitably pursued by such as 

 are prepared for it by a large amount of Geological and Minera- 

 logical knowledge ; and to follow it out systematically will require 

 a large expenditure of time and patience. And as the limited scope 

 of this Treatise forbids any more extended notice of it, the Reader 

 who desires further information as to what has been already done, 

 is referred to the sources mentioned below. * 



* See the Memoir of Mr. Sorby ' On some peculiarities in the Micro- 

 scopic Structure of Crystals,' in the "Quart. Journ. of the Geolog. 

 Society," Vol. xiv., p. 242 ; 'On the Microscopical Structure of Crystals, 

 indicating the origin of Minerals and Rocks,' Op. cit., p. 453; 'On 

 the original nature and subsequent alteration of Mica- Schist,' Op. cit., 

 Vol. xix., p. 401 ; the Memoir by Mr. David Forbes 'The Microscope in 

 Geology,' in the " Popular Science Review," Oct. 1867 ; and the Treatise 

 of Vogelsang " Philosophic der Geologie und Mikroskopische Gesteins- 

 studien," Bonn, 1867. 



