THE GEOLOGIST 



MAY, 1858. 



THE PHYSICAL STRUCTURE OF MERIONETHSHIRE AND 

 CAERNARVONSHIRE. 



By Professor A. C. Rimsat, E.R.S. and G.S. 



The view from the summit of Cader Idris is one of the most 

 magnificent in "Wales. Turning to the south, the landscape con- 

 sists of a series of rolling hills, stretching as far as the eye can 

 reach, in smoothly swelling outlines ; and it is impossible not to 

 be struck with the circumstance that, though some rise higher than 

 others, they maintain a sort of average height, accompanied by a 

 general gentle inclination towards the sea. Through this broad 

 undulation innumerable valleys have been excavated. The hills have 

 not been formed merely in consequence of disturbance of the strata ; 

 but, through the outcropping and smoothed edges of disturbed strata, 

 many great and small valleys have been cut by old marine denudations, 

 atmospheric disintegrations, and the action of running water. The 

 general uniform character of the outlines of this country is due to the 

 sameness of the rocks, which, in the main, consist of slates, occa- 

 sionally varied by hard or interstratified bands of sandstone, the whole 

 belonging to that part of the Silurian series known as the Caradoc or 

 Bala beds, and the rocks immediately overlying. 



Turning northwards, a change in the character of the landscape 

 becomes at once apparent. The mountains are broken and rugged. 

 The summits of the Arans, the great swelling mass of Rhobell-fawr, 

 the cliffs of Rhinog-fawr and Rhinog-fach, that guard the pass of 



