7 6 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Petrology, it is clear, could make but little progress until the 

 improvement of microscopic methods enabled us to make accurate 

 determinations of the minerals in a rock, even when these are 

 present as the most minute particles. The characteristic peculiarities 

 of the different rock-forming minerals, so carefully studied by 

 Zirkel, their accurate optical diagnosis, at which Eosenbusch has 

 laboured with so much success, supplemented by the micro-chemical 

 methods of Knop, JBoricky, Streng, and Behrens, and the pyro- 

 chemical method of Szabo, have already done much to render exact 

 our methods of recognizing the minerals in a rock. The contri- 

 vances, for which we are principally indebted to the French 

 petrographers, for effecting the isolation of the minerals in rocks, 

 so that they may be submitted to accurate chemical analysis, enable 

 us in cases of difficulty or doubt to confirm or check the results of our 

 microscopic studies. 



But there is at present perhaps a tendency to confound the end 

 with the means in such researches as these. When all the varieties 

 of minerals in a rock have been correctly identified, the work of the 

 Petrologist is not ended ; on the contrary, it is only just begun. 



The relationship of the several minerals in a rock to one another 

 the discrimination between such as are original and those of 

 secondary origin, and the recognition among the former of the 

 essential, as distinguished from those that are accessory or accidental, 

 — these are problems of even greater importance than the exact 

 determination of the species or varieties to which each belongs. In 

 not a few rocks it can be demonstrated that every one of its present 

 mineral constituents is different from those of which it was originally 

 made up ; in some cases, indeed, it may be shown that the recombi- 

 nation of the elements of the rock into fresh mineral aggregates has 

 taken place again and again. As well might we try to give a 

 rational account of our English speech without taking into account 

 the series of changes through which it has passed in its evolution 

 from the Anglo-Saxon dialects, as to explain the nature of a rock 

 without studying the influence upon it of the forces by which it has 

 gradually acquired its present characters. 



"With respect to the geographical distribution of the different 

 mineral species, many suggestive observations have been made. 

 Some, like the felspars, the pyroxenes, and the olivines, appear to be 

 ubiquitous in our earth's crust, and even make their appearance 

 again in those bodies of extra-terrestrial origin — the meteorites. 

 Others, like leucite, nepheline, hauyne, sodalite, and melilite, are 



