170 ME. F. E. COWPEE EEED ON THE GEOLOGY OE [May 1895, 



forces that determined the ultimate character and degree of per- 

 fection of the micropoikilitic structure. 



"With regard to the field-evidence, the size and nearness of the 

 intrusive masses seem connected with the degree of development of 

 this structure, the most perfect and complete examples of coarse 

 mosaic being found nearest the largest masses of diabase. An instance 

 of this is the Strumble Head set of felsites. Not a little seems 

 to depend on the original nature of the rock, as might be expected. 

 The existence of intrusive masses concealed underground mav 

 account for cases otherwise difficult to explain. 



I would still ascribe the infilling of the vesicles and veins with 

 quartz to a different and subsequent process ; the abundance of 

 secondary quartz (see p. 183) in some of the diabases lends support 

 to this view. The sort of solution that caused this infilling must 

 have been in a very different state from the highly heated one 

 which produced the micropoikilitic structure. On the whole I 

 think we may regard the latter structure as a result, at any rate 

 in many cases, of contact-metamorphism. 



Mr. Hutchings (Geol. Mag. 1894) mentions the occurrence of this 

 structure, which he calls the ' tesselated or mosaic structure,' in 

 certain altered andesitic and rhyolitic ashes in the contact-zone of 

 the Shap granite, and says that it is by no means peculiar to the 

 alteration of sedimentary rocks. 



A rhyolitic tuff [58] which is in contact with a mass of diabase 

 on Fishguard Quay (on the western side of the harbour) exhibits the 

 micropoikilitic structure well developed. This is an important con- 

 firmation of the previously expressed views. 



The same author further reminds us that Lieut.-Gren. M c Mahon 

 described a similar structure on the margin of the Dartmoor 

 granite, 1 and previously in granite of the Himalayas. In the 

 groundmass of certain quartz-porphyries it is well known, 2 and 

 Mr. Hutchings says that there it is indistinguishable from the finer 

 mosaic of slates, except by the characters of the inclusions. On 

 Castle Point, Fishguard, it is difficult to say whether a rock [68] in 

 which a small patched mosaic is developed was originally a fine 

 tuff or a lava-flow, but the general character of the inclusions leads 

 one to consider it a tuff. 



The somewhat erratic manner in which the mosaic structure is 

 developed may be explained, as the same writer suggests, by the 

 irregular contour of the intrusion below ground, and also by the 

 varying degrees of susceptibility of the more or less different rocks. 

 Naturally one would have liked to trace the series of stages of its 

 development in one continuous bed of lava. But the nature of the 

 ground and the variability of each lava-flow did not allow of this 

 being done in any case with which I met. In the absence, therefore, 

 of definite field-evidence of this kind I must leave it, at any rate for 

 the present, an undecided question whether Mr. Hutchings's expla- 



1 Quart. Joum. Geol. Soc. vol. xlix. (1893) p. 388. 



2 Haworth, ' American Geologist,' vol. i. (1888) p. 363. 



