544 J. R. Bahyns 8f E. Greenly — Fekitic Slates of Snoivdon. 



is the utmost difficulty in fixing on a boundary-line between it and' 

 the overlying ash, both being felspathic and both cleaved and 

 jointed. It is difficult to recognise the probability of intense^ 

 cleavage being induced in a rock that has been melted, accustomed 

 as we are to consider that cleavage of this nature is confined to 

 stratified rocks. Less than two miles from this spot, on the sides 

 of Cwm-y-llan, near Yr Aran, the same structure is remarkable, 

 and it was not till after repeated visits and much study on the spot 

 that Mr. Selwyn was enabled to draw the lines of demarcation 

 between the ashes and the felspathic porphyry." 



Mr. Harker, in describing the rhyolitic lavas, refers merely to an 

 'incipient' or 'imperfect' cleavage/ distinguishing it from the fine 

 platy jointing and fluidal structure ; and it is evident, therefore, 

 that he is not referring to these rocks, in which the cleavage is 

 correctly described by Eamsay as ' intense.' 



Evidence of Clastic Origin. 



In this Magazine for June, 1900, p. 2G8, in a paper on " First- 

 fruits of a Geological Examination of Snowdon," Mr. Dakyns 

 described these felsitic slates, distinguishing them from the other 

 components of the lower felstones. He pointed out that in many 

 places they can be seen to be fragmental (giving localities), and 

 that these obviously fragmental rocks are part and parcel of the fine 

 felsitic slates. He also drew attention to the absence of bedding, 

 a character which we shall consider further on. We may add, 

 perhaps, that were these rocks felsitic lavas, abundant traces should 

 be found of the fluidal and also of the spheroidal structui-es that are 

 so frequent in the true lavas. Cleavage would not have obliterated 

 these throughout so large a mass of rock. In the paper quoted some 

 microscopical evidence was also adduced, the most important part 

 of which was that the felsitic slates contain a considerable quantity 

 of calcite, whereas the felsitic lavas have in no case'" yielded more 

 than '34 per cent, of lime.^ 



Ee-examination of the same material has shown that some of the 

 "small aggregates with igneous structure" are really felspars which 

 have undergone a peculiar kind of alteration. But to this we shall 

 revert in describing the metamorphism. Besides these, however, 

 there are oval bodies consisting of felspars bound together by iron 

 oxide and indeterminate brown matter, which probably represents 

 a more basic glass, and these appear to be genuine lapilli. The fine 

 matrix is far too much deformed and indeed reconstructed to retain 

 any trace of its original texture, except under the sheltered ' lee ' 

 of some of the larger felspars, and in these parts smaller, subangular 

 felspars can be seen which have the aspect of broken grains. 



The conclusion arrived at by Mr. Dakyns in his paper of June, 

 1900, was that the rock was a felsite tuif or dust. 



1 " Bala Volcanic Series," pp. 16, 17. 



2 Harker, " Bala Volcanic Series," p. 13. 



^ The average of eight analyses of ancient and modern rhyolites given in Sir A . 

 Geikie's " Text Book of Geology," 4th ed., 1903, p. 212, is -698. 



