186 ME. E. E. COWPEE EEED ON THE GEOLOGY OF [May 1 895, 



Miss Eaisin's second type {op. tit. p. 152) are not present in any 

 instance. 



I am q'uite in accord with Miss Eaisin's view that the variolitic 

 structure is not necessarily developed at the exterior of the intrusive 

 mass, and is not always connected with the exterior of spheroids. 

 In the case of Garn Eechan, for instance, the whole ' Garn' 

 from its northern to its southern borders has the incipient varioles 

 more or less developed, without reference to the propinquity of the 

 margins. 



Yariolite was defined by Cole and Gregory as a ' devitrified 

 spherulitic tachylyte, typically coarse in structure.' x The association 

 of the devitrified tachylytic rock of Garn Eawr, which forms in 

 itself a huge mass, and is the western termination of the intrusive 

 belt that stretches from Pwll Deri to Pont Iago, with the adjoining 

 smaller variolitic mass of Garn Eechan, which merges insensibly 

 into the Garn Fawr mass and certainly constitutes part of the 

 same intrusion, suggests strange conditions of cooling and crystal- 

 lization. We must get rid of the impression produced by the present 

 configuration of the locality in order to form an accurate idea of the 

 actual circumstances at the time of the intrusion. A probable 

 explanation seems to me that the molten material was forced between 

 highly tilted strata in such a way that merely one end of the laccolite 

 approached the surface while the rest of it was deeply buried beneath 

 the superincumbent strata, and that owing to its nearness to the 

 surface of the ground, or to the composition or structure of the beds 

 surrounding this end, or to all combined, this portion of the laccolite 

 cooled with considerable rapidity. It has been pointed out by 

 Miss Eaisin that spherulitic structures tend to form in parts 

 where the cooling is not rapid enough to give rise to a homogeneous 

 glass, nor slow enough to produce well-formed crystals. 2 It is just 

 in this intermediate but indefinitely-bounded zone that we find the 

 varioles occurring at Garn Eechan. 



In all these variolitic and tachylytic rocks the forms of the 

 felspars are of special interest. Perfect crystals are rare, and occur 

 porphyritically. Skeleton-felspars are exceedingly common, and 

 fork-shaped terminations are often seen. The bent and wavy 

 felspar-microlites have generally a dark granular core, which repre- 

 sents an included portion of the undifferentiated magma and is not 

 due to secondary alteration. The fine fibre-like microlites are often 

 of considerable length. The shorter microlites are grouped in tufts 

 round the ends of the porphyritic felspar-crystals, or form a fringe 

 round them. Between this condition and that of independent 

 axiolitic or spherulitic groups there is every gradation to be found. 

 One may therefore infer that the ' incipient varioles ' are here inti- 

 mately connected with the second growth of felspar-crystals. And 

 that they were formed comparatively early in the consolidation of 

 the rock appears indicated by the fact that the fibrous fringe 



1 Quart. Journ. Geol. Soc. vol. xlvi. (1890) p. 330. 



2 DM. vol. xlix. (1893) p. 156. 



