J. B. Dahjns ^ E. Greenly — Felsitic Slates of Snowdon. 547 



felspars also show a tendency to break up at their edges into a 

 mosaic. It is evident that the rectangularly bounded aggregates 

 are really reconstructed felspars, and it is therefore probable that 

 the lenticular aggregates are also felspars which have been deformed 

 externally as well. 



Fig. 3. — Felspar Lenticle in Felsitic Slate. About "75 mm. 

 Fig. 4. — Eeconstructed Felspar in Felsitic Slate. About "5 mm. 

 In both figures the shaded bands are twin lamellae. 



This reconstruction of felspar is most interesting, particularly as 

 it is evidently here a result of dynamic metamorphism, and as the 

 resulting product shows no trace of mechanical crush or strain. 

 Indeed, the extent to which mineral reconstruction has been carried 

 is remarkable, when we reflect that no one would have called any 

 rock of the series a ' crystalline schist ' ; that the results can be 

 ascribed to no other agency than that which has impressed the 

 slaty cleavage ; and that this agency did not come into operation 

 until after the Ordovician period. 



JExceptional Nature and probable Origin of the BocTcs. 



From the evidence given in this paper it is clear that volcanic 

 dust forms a large part, even if it does not form the whole, of the 

 felsitic slates of Snowdon. 



But the most remarkable feature of the deposit is the exceeding 

 rarity of any signs of bedding. Intense though the cleavage is, 

 it could not have obliterated this, nor indeed has it done so in the 

 ashy series above or the sedimentary series below. And ages of 

 weathering on the exposed faces of lofty cliffs could hardly have 

 failed to reveal bedding if any existed : but in Llechog and 

 other great crags nothing can be seen from top to bottom but 

 the uniform vertical divisional planes which determine every feature. 

 Yet such a mass of dust, if projected in the ordinary manner, and 

 falling as a rain of ashes, could not fail to become stratified, even in 

 the open air, much less on a sea bottom. The overlying ashy series 

 is well stratified, and yet it is composed of much coarser material. 

 Evidently, therefore, the felsitic slates, if a dust, must be due to 

 some unusual kind of eruption. 



