GABBROS ETC. IX SCOTLAND AND IRELAND. 51 



Two years later, a study of the district of Schemnitz, in Hungary, 

 enabled me to prove that what is true of the more basic and acid 

 types of rock, respectively, is equally true of the rocks which have 

 an intermediate composition (" roches neutres " of the French geolo- 

 gists). I showed that in Hungary, rocks which were commonly 

 spoken of as " syenites " and " granites " — but which really belong 

 to the diorites and quartz-diorites — are not only of Tertiary age, but 

 graduate insensibly through the so-called "propylites" and "green- 

 stone-trachytes" into the andesites and quartz-andesites, which 

 sometimes assume a perfectly vitreous condition*. 



These conclusions concerning the acid, the intermediate, and the 

 basic classes of rocks, respectively, were arrived at mainly by the 

 study of the rocks in the field ; but the observations were confirmed, 

 controlled, and checked at every point by the examination of trans- 

 parent sections of the rocks with the microscope. 



Although these conclusions have met with a very general accei)t- 

 ance among my fellow-workers in this country, it would be idle to 

 conceal from myself the fact that they have found but little favour 

 among the petrographers of Europe t. In spite of the support 

 which the views I have advocated have received from Professor 

 SiissJ and Dr. Reyer§, of Tienna, and from Dr. H. H. Reuschj], 

 of Christiania, the absolute distinction in age between the highly 

 plutonic and the volcanic types of igneous rocks has been maintained 

 and strongly insisted upon in nearly all the valuable petrographical 

 treatises and monographs which have appeared in Germany and 

 France since the date of the publication of those papers. Such 

 being the case, I have felt it to be my duty to carefully review 



* Quart. Journ. Geol. Soc. vol. xxxii. (1876), pp. 292-82-!-. See also ' Vol- 

 canoes : what they are and what they teach ' (1880), pp. 198-200. 



t In the year 1876, Professor Zirkel met my contention with respect to the 

 propyhtes or " greenstone-trachytes" of the older authors, by endeavouring to 

 show' that constant and recognizable differences existed between those i-ocks when 

 compared with the diorites on the one hand, and the andesites on the other 

 (U. S. Geol. Expl. of 40th Parallel, vol. vi. pp. Ill, 112). These views were to 

 some extent also supported by Professor Vom Eath. But, on the other hand, 

 Dolter, in 1879 (Verhandl. der k. k. Reiehsanstalt, 1879, p. 27), felt compelled to 

 adunt that rocks certaiuly occur in which the supposed distinction failed to 

 apply; and Szabo has also argued against propyhte being regarded as a distinct 

 rock-species (Yerhandl. der k. k. Reichsanstalt, 1879, p. 18 &c.). The same 

 view was maintained with respect to the American "propylites" by Dr. 

 Wadsworth in 1879 (Bull. Mus. Comp. Zool. Camb. Mass. vol. v. p. 285) and 

 1881 (Proc. Bosfc. Soc. Nat. Hist. vol. xxi. p. 2), and with regard to the Hungarian 

 ones by Dr. Koch, of Klausenburg, in 1880 (Verhandl. der k. k. Reichsanstalt, 

 1880). Most of the geologists of the United States Geological Survey, had fol- 

 lowed Richthofen and Zirkel in regarding propylite as a distinct rock-species, 

 which was erupted at a definite geological period : but in 1882 Mr. G. F. Becker 

 was able to announce the results which he had arrived at by long and careful 

 study in the field and in the laboratory. His conclusion was that the rocks in 

 question have no real claim to be ranked as a distinct rock-species, but are really 

 members of the diorite-andesite series (U. S. Geol. Survey, Monograph III., 

 Geologv of the Comstock Lode). 



X Anthtz der Erde (1883), pp. 204-206. 



§ Beitrag zur Fisik der Eruplionen und der Eruptivgesteinc (1877) p. 135 &c. 



11 H. H. Reusch, 'Ueber Vulkanismus' (Berlin, 1883). 



e2 



