ORIGIN OF THE ROCKS AND SCENERY OF NORTH WALES. 219 



a tendency to split into layers of indefinite thinness, the surfaces of which 

 are strictly parallel. Here and there are greenish patches in the purple 

 slate ; when exposed on the surface of a cleavage plane they are 

 slightly elliptical, but when exposed on the surface of a plane, at right 

 angles to the cleavage planes,|they are seen to be enormously drawn out, 

 with their longer axes in the direction of the cleavage planes. There can be 

 little doubt that the original shape of these green spaces was spherical, 

 and that their present shape has been produced by flattening in a 

 direction at right angles to the cleavage planes, and expansion in the 

 direction of these divisional lines. Experiment and observation both 

 combine to establish the theory that slaty cleavage is due to pressure, and 

 this distortion of what were originally green spheres enables us to 

 estimate roughly the amount of compression which has taken place. The 

 slates worked in the neighbourhood of Llanberis are usually of a purple 

 or green colour. The same band is worked at Moel Tryfan, near Bettws 

 Garmon, and at the celebrated Penrhyn quarries, in the valley of Nant 

 Francon. 



We pass now to the consideration of a class of rocks very different 

 from those just described. The exact determination of the origin of 

 many of these rocks is extremely difficult, owing to the enormous altera- 

 tion they have undergone during the lapse of ages. 



The adjective " Felspathic" is usually applied to this class, owing 

 either to the actual presence of felspar crystals, whether perfect or 

 fragmentary, or to the similarity in chemical and physical aspect between 

 the i-ocks of this class and the felspar group of minerals. When the rock 

 is fine grained and compact it is frequently termed a felstone, but there 

 is no guarantee that all rocks to which this term is applied have been 

 produced in the same manner. These felspathic rocks are of great 

 thickness and extent. The more mountainous districts, including of 

 course the great Snowdon group, are almost entirely composed of them, 

 and owe their rugged grandeur to the resistance which they offer to the 

 agencies of waste. Although a vast amount of research will have to 

 be expended on these rocks before anything like a complete account of 

 their origin can be given, still I think the main conclusion of Professor 

 Ramsay, as stated in Vol. LTI. of Survey Memoirs will hold true. These 

 conclusions may be stated as follows : — 



(1) That these felspathic rocks are mainly the products of volcanic 

 action. 



(2) That they belong to three distinct classes : — 



a Fragmental volcanic rocks. 



b True lavas. 



c Intrusive rocks. 



(3) That the ashes and lavas were poured out for the most part 

 beneath the sea. 



(To be continued.) 



