Cole— 0^? the Growth of Crystals. 123 



microcline and quartz ; and sphene and allanite again are present. 

 The quartz has developed in this rock in spheroidal blebs in advance 

 of the microcline, which thus encloses it ophitically. Something 

 similar to this relation between quartz and microcline has been noted 

 by Prof. Sollas in the soda-granites of the Leinster chain J 



Of greater interest, near Summy Lough, are the lumps of earlier 

 diorite and garnet-amphibolite enclosed in the later basic rocks, both 

 series being now modified by granite veins. The earlier diorite 

 consists of hornblende, magnetite, and basic labradorite, and weathers 

 away more easily than the later one, leaving little pits upon the 

 surface. I have elsewhere- commented on the similarity of these 

 features to those described by Mr. Harker from the far more recent 

 and more volcanic series which has been so admirably investigated by 

 him in Skye. 



It will be seen, then, that the coarseness of grain of a composite 

 rock cannot depend upon conditions of pressure alone, but that a 

 certain stimulus is given to the recrystallisation of older basic 

 material, be it amphibole or garnet, by the invasion of a molten 

 granite mass. This mass, when it finally consolidates, may itself 

 remain of medium grain ; it may therefore be conceived that the 

 continuous passage of heat outward from it into the contact-zone 

 maintains the rock attacked in a favourable condition, and promotes 

 the growth of certain mineral species. The contrast between this 

 effect and the production of a granular mosaic in ordinary epidio- 

 rites" enables us in some degree to judge between the results of 

 igneous and dynamic metamorphism. It is obvious from the form and 

 grouping of the later amphiboles of Cor and Cashel Hill that no 

 dynamic changes of importance have taken place in the area since the 

 intrusion of the granite which stimulated their growth. 



^ " Preliminary Account of the Soda-granites of Co. Wicklow," Sci. Proc. 

 R. Dublin Soc, vol. vi. (1889), p. 267, and "Granites of Leinster," Trans. 

 R. I A., vol. xxix. (1891), p. 509. 



- " The Heart of Skye," Nature, vol. Ixx. (1904), p. 507. 



R.I. A. PROC, VOL. XXV., SEC. K.] 



M 



