366 PROF. J. W. JFDD OX THE PEOPYLITES 



VII. Causes by which the " Propylitic " Modification of these 



KOCKS HAS been brought ABOUT. 



In studying the relations of the propylites to the other rocks of 

 the Western Isles of Scotland no fact strikes the observer more 

 forcibly than that of their being constantly invaded by igneous in- 

 trusions, composed of granite and felsite on the one hand, and of 

 gabbro and dolerite on the other hand. Beautiful examples may- 

 be studied, at many points, of the ramification of granite veins 

 through the propylites, and portions of the propylite may even be 

 seen caught up in the midst of granite. On the other hand, the 

 currents of propylite may be seen to be broken up by numerous sheets 

 of gabbro and dolerite, which are generally intruded between them ; 

 and from these sheets numerous veins and dykes of dolerite and 

 basalt can often be traced intersecting the propylite masses. In 

 places the intrusions, indeed, outbulk the rocks among which they 

 have been thrust. These relations were fully explained by me in 

 1874 *, and have been confirmed by numerous illustrative examples 

 given by Dr. A. Geikie in 1888 t. 



Impressed by the number of these intrusions among the pro- 

 pylite- (" felstone "-) lavas I was led, in 1874, to refer the re- 

 markable alteration which they have undoubtedly undergone to 

 contact-metamorphism J, and the same view was adopted by Dr. 

 A. Geikie in 1888 §. 



But subsequent and more detailed study of the propylites has 

 convinced me that contact-metamorphism, while producing very 

 striking results close to the planes of junction of the lavas and the 

 intruded sheets, has seldom operated to any great distances from the 

 latter, and that the widespread modifications which have been 

 eff'ected in the minerals and the ground-mass of the propylites must 

 be referred to a widely different cause. In the case of the analogous 

 rocks of Eastern Europe and i!^orth America, it has been abundantly 

 proved, by the researches of geologists in these districts, that the 

 cause of the curious alteration of the andesitic and dioritic rocks 

 and their impregnation with metallic sulphides (some of which are 

 of great commercial value in those two districts) must be referred 

 to the action of steam and of various acid gases, which have per- 

 meated the whole substance of the lava-masses, giving rise to pro- 

 found chemical alteration of their constituents. 



At the same time the contact-metamorphism, to which I called 

 attention in 1874, has undoubtedly been a noteworthy and, in some 

 instances, an important contributory factor in bringing about the 

 results. I showed that " in proximity to the gabbros, these felstone- 

 lavas are seen ... to have acquired a peculiar platy structure and 

 splintery fracture, combined, in many cases, with the development 

 of a probably preexisting banded coloration " ||. I particularly dwelt 



* Quart. Journ. Geol. Soc. vol xxx. p. 246. 



t Trans. Roy. Soc. Eclinb. vol. xxxv. (1888) pp. 165-171. 



\ Quart. Journ. Geol. Soc. vol. xxx. p. 251. 



§ Trans. Roy. Soc. Eclinb. -vol. xxxv. (1888) p. 167. 



II Quart. Journ. Geol. Soc. vol. xxx. p. 251. 



