344 



DR. E. GREENLY ON THE SUCCESSION AND [vol. lxxix, 



the other. It must, therefore, have possessed sufficient cohesion to 

 hold the groups together, which could not have been the case until 

 consolidation was very far advanced, or, possibly, complete. 

 Further, in thoroughly-banded, and even folded, gneisses all over 

 the Allor area, survivals of these peculiar hornblende-groups can 

 usually be found on scrutiny. Such gneisses, therefore, must have 

 been produced by the rolling-out of a differentiated magma, and 

 at a very advanced stage of crystallization. 



Fig. 4. — Pegmatites luitli foliated ultrabasic encasements in basic 

 gneisses: road-fork south-west of Clegir-mawr ; and Craig- 

 allor (y i|). 







Pegmatites. — Three generations of these have now been 

 distinguished. It is with the first and second generations that Ave 

 are concerned here. Most of them are composed almost wholly of 

 albite, often beautifully twinned ; but the wider veins contain 

 hypidiomorphic hornblendes, occasionally measuring as much as an 

 inch in diameter. Pegmatites with a width of 2 or 3 inches, 

 however, are rare, the majority being less than half-an-inch, while 

 many are no more than a quarter of an inch thick. Despite the 

 fact that their crystals interlock with those of the gneiss, they do 

 not pass gradually into it, but are defined in the sharpest manner, 

 their margins being clear even under the microscope. By far the 

 greater number of them conform to the foliation of the gneiss 



