J. B. Dakyns 8f E. Greenly — Fehitic Slates of Snoicdon. 545 



Fresli Evidence. 



In April, 1900, Mr. Dakyns, while seeking for a good line of 

 division between the calcareous ashy series and the underlying 

 felsitic rocks at Hafod Llwyfog, near Llyn Gwynant, found, in 

 a band of felsitic slaty rock, some Brachiopoda, the best preserved 

 of which were StropJiomena and Orthis, though Mr. E. T. Newton, 

 F.R.S., who kindly examined these fossils, hesitated to name the 

 species. Of the clastic origin of this rook there could therefore be 

 no doubt. 



To the unaided eye, or with a pocket lens, it has the same aspect 

 as the other felsitic slates, though somewhat less fissile. If any- 

 thing, it looks rather more like the felsitic lavas than they do. 



Under the microscope, it is seen to be somewhat less altered, and 

 the original character of even the finer parts can still be made out 

 in places. It consists of the same small felspar crystals, set in 

 a fine grey matrix, with streaks of calcite dust, which would 

 account for the calcite before noted in the more altered varieties. 

 There is also some chlorite. This matrix can be seen to be un- 

 doubtedly clastic, and consists of very small grains of felspar 

 (quartz has not been identified), often of very irregular form. In 

 other parts of the slide nothing but aggregate polarisation can 

 be seen. 



The most singular feature, perhaps, of the rock is the presence of 

 a number of small flakes of biotite, with strong pleochroism, rich 

 brown to nearly colourless. These are sometimes isolated, but more 

 often in groups (Figs. 1 and 2), in which they and small felspars are 



Figs. 1 and 2. — Groups of Biotite Crystals in Fossiliferous Felsitic Eock. 

 Highly magnified (about '23 mm.). 



bound together by matter partly felsitic (giving aggregate polarisation) 

 and partly still isotropic. The biotite flakes lie in all directions, 

 and are evidently original constituents. Their arrangement suggests 

 that these groups were originally lapilli, in which the biotites and 

 felspars were cemented by a certain amount of glass. Most of the 

 chlorite of the rock can be seen to be due to the alteration of biotite. 

 Biotite in an unchanged condition has not, so far as we remember, 

 been described from the Felsitic Series of Snowdon. 



This rock, therefore, is a dust, mainly felspathic, but containing 

 scattered lapilli of a somewhat more basic character. 



BECADE V. — VOL. II. — NO. XII. 



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