Geological Society. 279 



tional evidence was now brought forward concerning the nature of 

 the processes by which these structures — variously known as the 

 micropegmatitic, the centric or ocellar, the pseudospherulitic, the 

 microgranitic, and the drusy or miarolitic — which are found in the 

 peripheral zones and the apophyses of granitic intrusions, must 

 have been produced. 



That fragments of crystals in detrital rocks undergo enlargement 

 and redevelopment has been shown by Sorby, Van Hise, Bonney, 

 and many other authors. The fact has also been frequently re- 

 cognized that curious outgrowths may often be detected in con- 

 nexion with the crystals of igneous rocks ; such outgrowths have 

 usually been regarded, however, as having been formed during the 

 original consolidation of the rock. 



In a " labradorite-andesite " (labradorite of French petrographers) 

 belonging to the older or " felstone " series of ejections in the Ter- 

 tiary volcano of Mull, large crystals of a plagioclase-felspar, near to 

 labradorite in composition, are found to exhibit large and re- 

 markable outgrowths of very irregular forms. The distinction 

 between these outgrowths and the original crystals is rendered very 

 obvious from the circumstance that the original crystals have been 

 corroded by the enveloping magma and contain enclosures of the 

 same, and that they have been much cracked, and sometimes even 

 partially kaohnized before growth recommenced in them. In some 

 cases the crystals have been actually broken and recemented by 

 newly deposited felspar-material. 



While there is a general crystailographic continuity between the 

 old felspar-crystals and the new outgrowths from them, the varia- 

 tions in the position of extinction in different portions of the en- 

 larged crystal show that, as growth went on, the composition of 

 successively formed zones gradually and progressively changed from 

 near the Anorthite limit to close upon the Albite limit. 



These facts prove that, under suitable conditions, felspar-crystals 

 in solid rock-masses may grow at the expense of the unstable glass- 

 magma by which they are surrounded. This conclusion is in com- 

 plete harmony with some other recent researches — especially those 

 of Dr. J. Lehmann on the mode of production of the perthite- 

 structure in felspars. In conclusion, the circumstances which have 

 given rise to the exceptionally clear illustration of the processes 

 described in the rock under consideration were explained, and the 

 bearings of the principles enunciated on the theory of metamorphism 

 are indicated. 



2. " The Tertiary Yolcanoes of the Western Isles of Scotland." 

 By Prof. J. W. Judd, F.R.S., F.G.S. 



In bis recently published memoir, " The History of Volcanic 

 Action during the Tertiary Period in the British Isles," Dr. A. 

 Geikie, while adopting many of the views propounded in a commu- 

 nication made to this Society in 1874, " On the Ancient Volcanoes 

 of the Highlands," takes exception to certain of the conclusions 

 which are maintained in that paper. 



