THE EVOLUTION OF PETROLOGICAL IDEAS. 291 



PROGRESS DURING -T^HE FIRST HALF OF THE CENTURY. 



Igneous rocks played but a suiall part in the Wernerian system. 

 They were regarded as stratified rocks melted by heat, due to the 

 burning of coal ])eneath volcanic districts. We now recognize that 

 they are of great importance, and probably represent the original 

 source of all the other ro.cks. The clearing up of our ideas as to their 

 nature and mode of origin centers round two controversies — one as to 

 the origin of basalt, the other as to the origin of granite. 



It is difficult for us to realize the condition of things which prevailed 

 during the early years of the century, when the martial spirit of the age 

 seems to have affected the scientific world and a furious controversy 

 raged between the Neptunists and the Yulcanists as to th(> origin of 

 l>asalt. We look with a feeling of astonishment at the controver- 

 sialists, condenm their methods, and admire the calm tigure of the old 

 man Desmarest as he sits there refusing to be dragged into the con- 

 troversy, and ([uietly replies to his challengers, ''Go and see." Now 

 and again, in looking through some neglected cabinets of our nmseums, 

 we come across dust-covered specimens labeled " Annnonites in basalt 

 from Portrush,'' and are thus forcibly reminded of those stirring times. 



The controversv a^s to the origin of granite lasted hunger, and (hiring 

 its later stages, at any rate, was conducted with dignity and a due 

 regard to the amenities of scientific discussion. It resulted, moreover, 

 in a very decided enlargemtmt of our conceptions as to su])terranean 

 phenomena. The Wernerian \'iew that granite was a precipitate from 

 a primordial ocean was compelled to give way as soon as the tectonic 

 relations of granitic masses, so well described l)y Hutton, Playfair, 

 and Sir James Hall, were clearly realized. The phenomena of granite 

 veins, the occurrence of inclusions of the surrounding rocks, and the 

 sharpness of the junctions between granite and the strata with which 

 it is in contact prove beyond all doubt that th(^ material of which 

 many granite masses are conjposed nmst have l)een intruded from 

 l)elow in a plastic state. Toward the middle of the century these facts 

 were generally recognized for all those masses \vhich occur outside 

 areas of ciystalline schist. But their recognition, although conclusive 

 as against the view that granite was eveiywhere a ])rim<)rdial sediment, 

 by no means involved the necessity of accepting the Huttonian theor}' 

 that it resulted from the consolidation of a mass of matter in a state of 

 pure igneous fusion. 



The earlier phases of the discussion as to the origin of granite cen- 

 tered mainly round the tectonic relations of the rock masses; but the 

 later phases had reference rather to the composition and structure of 

 the rock itself. The ])apers on this subject, and especially th(> discus- 

 sion between Scheerer and Durocher, are well worthy of the attention 

 of modern petrologists. 



