THE SYENITE OF BRADGATE PAKK. 107 



tail one into the other, and the felspar occasionally exhibits in the 

 thin sections what are either true inclusions of quartz or sections 

 of lobe-like prominences of that mineral. That is to say, we find 

 in certain vein-granites, though on a smaller scale, the nearest re- 

 semblances to a structure which, so far as I know, is characteristic of 

 the granitoid gneisses of the older Archaean. It appears, then, to me 

 that this peculiar structure, which in the case of vein-granites may 

 be traced into the normal structure of granite with fairly idiomorphic 

 felspars, indicates some mode of what I may call " constrained 

 crystallization ; " that is to say, the mass did not possess the complete- 

 freedom of molecular movement which has existed in the case of a 

 normal granite. Probably at first it lost heat somewhat rapidly, and 

 so rather quickly assumed a "pasty" condition. The same cause — 

 constraint, due to the mass having previously solidified — may explaio 

 the peculiar confused indeterminate structure which is more strictly 

 called cryptocrystalline, for, as I have elsewhere shown, both from 

 my own investigations and those of others, it is evident that con- 

 siderable separation of constituents may take place without actual 

 melting *. Perhaps cases often have occurred in nature where the 

 mass has not even become plastic, but the action of pressure and 

 water, at a low temperature, has caused the constituents to enter 

 into more stable combinations, though only a small amount of 

 molecular movement has been possible. Thus, in some devitrified 

 rocks it is hardly possible to recognize individual crystals, or say 

 more than that some minute quartz appears to have segregated 

 generall}' from the originally vitreous mass, the residue of which is 

 a silicate, usually, if not always, crystalline, and in most cases a 

 felspar. 



Discussion on the above two Papers. 



Prof. Blake was glad to hear that the Peldar-Tor rock was defi- 

 nitely admitted to be igneous. On his last visit to Bardon HiU he 

 had been struck with the evidence of crushing. With regard to 

 what the Authors had inferred to be a pyroclastic rock at Bardon 

 Hill, he believed the same arguments would hold good as those which 

 had been advanced in the case of Peldar Tor. 



Dr. Callaway had been puzzled in trying to distinguish pyroclastic 

 rocks from igneous ones in Shropshire. He had not been able to 

 find the slightest evidence of the former presence of an ice-sheet in 

 that county. 



Mr. J. W. Gregory called attention to the limited amount of 

 alteration described by Lacroix in the Trenton Limestone in Canada, 

 when in contact with the pegmatitic as compared with the normal 

 nepheline-syenite. 



Gen. McMahon said he was glad to add his testimony in corro- 

 boration of the conclusion arrived at by the Authors regarding the 

 Sharpley and Peldar-Tor rocks. In 1888 he collected good speci- 

 mens from both localities, those from Sharpley having been selected 



* Pres. Address, 1885, Proc. Geol. Soc. p. G5, &c. vol. xli. 



