Forbes — The Microscope in Geology. 513 



trachytes, dolerites, diorites, porphyrites, syenites, granites, etc., all 

 possess the same general structural features, serving to distinguish 

 the eruptive rocks as a class from all others. 



In the examination and discrimination of the minerals which 

 compose these rocks, especially when close-grained, the microscope 

 is quite indispensable, since without it no such enquiry could be 

 attempted. In these examinations the assistance of polarised light 

 is most valuable ; but the space, unfortunately, only allows of a 

 mere mention of its application. In distinguishing dolerites from 

 diorites, when fine-grained (as is often of considerable geological 

 importance) , the fibrous structure of the hornblende of the latter is 

 generally so well developed, even when present in very minute 

 quantity, as to distinguish it readily from the augite of the former, 

 which possesses no such structure. Even in the case of Uralite, 

 a mineral characteristic of certain porphyritic rocks, which has the 

 external form of augite, although its chemical composition is that of 

 hornblende, the fibrous structure characteristic of hornblende is 

 distinctly visible. The microscopic structure of some minerals, 

 however, varies with their origin ; thus Sorby has shown that the 

 structure of augite, and some other minerals in meteorites, is quite 

 distinct from that of the same minerals occurring in eruptive rocks, 

 and demonstrated, in a very striking manner, how the study of such 

 peculiarities is likely to clear up the mystery in which the origin of 

 these bodies is involved. 



When, as is often the case, especially with translucent, colourless 

 minerals like quartz, leucite, calcite, felspar, etc., the appearance 

 presented under the microscope is alike, their optical properties and 

 the use of polarised light afford the means of distiuguishing between 

 them with certainty ; as, also, in the event of one substance being 

 present under two forms, as calcite from aragonite, monoclinic from 

 triclinic felspars, etc. In a similar manner, the structure, whether 

 crystalline or vitreous, is determined, and valuable information 

 obtained, elucidating the mode of formation and origin of the rocks 

 themselves. 



The alterations produced in eruptive rocks subsequent to their 

 solidification, by the action of water, atmospheric, or other agencies, 

 are studied with advantage under the microscope. 



In a section given, the skeleton of labradorite is seen remaining as 

 evidence of the original crystallised structure, whilst the interstices 

 contain the products of the alteration of the more easily decompos- 

 able augite, the structure of which is nearly obliterated, and part of 

 its lime converted into carbonate. The rock in question is the 

 so-called " white horse " of Staffordshire, found imbedded in, or 

 breaking through the Coal-measures, which are frequently burnt or 

 altered at points of contact with this rock, which itself often has the 

 appearance of a whitish clay. The origin of this rock, whether 

 sedimentary or igneous, was disputed until the more recent geologi- 

 cal and chemical examinations of it have proved satisfactorily its 

 identity with the Kowley basaltic rock, very similar to that of 

 PoukhiU. 



VOL. IV. — NO. XLI. 33 



