part 2] ITS STEUCTURE AND ROCK-SFCCESSIOlSr. 135 



but, later in the Geological Survey Memoir on North Wales (2nd 

 eel. 1881), he gives 11,000 feet as a more probable figure, and 

 states that the throw diminishes north-westwards in tbe region of 

 the lake to 5000 feet. The angle of the fault, as drawn, shows a 

 steep north-north-westward dip, but it is admitted that the angle 

 is hypothetical. 



Philip Lake, in his paper on ' Bala Lake & the River-Sj^stem 

 of North Wales,' ^ deals with the faults in the region south-west 

 of the lake, and considers that there are two faults traversing the 

 lake, one parallel to the south-eastern shore (though some little 

 distance within the water's edge) and the other parallel to the 

 north-western shore. 



In his paper on the ' Composition & Structure of the Hirnant 

 Limestone,' L. W. Fulcher^ points out that the grains in the 

 rock are ellipsoidal in shape, measuring 1 to 3 mm. in their longest 

 diameter, and they are more sparsely scattered in the crystalline 

 matrix than in an ordinary oolite or pisolite ; he also notices that 

 the blackness of the grains, which is so marked a feature of the 

 rock, is due to the presence of carbon in an amorphous form. 



II. Geneeal Physical Featuees. 



The country is essentially a hilly one : on the south-east of the 

 lake the ground rises steeply from lake-level (530 feet) to 1000 feet, 

 the steepest part of this rise appearing to coincide with a well- 

 marked line of fault ; above the altitude of 1000 feet, however, 

 the slopes are gentler and less regular up to the summit of Moel- 

 fryn (1750 feet) ; beyond the summit the ground falls somewhat 

 rapidly into the steep-sided Hirnant Valley, but rises again beyond 

 to a height of about 2000 feet. On the north-west side of the 

 lake the rise is also steep to the 900-foot contour-line, and is 

 probabl}^ determined by another fault-line. Above this height 

 there is a marked contrast with the opposite side of the lake, for 

 there is a wide stretch of mereh'' undulating moorland with scarcely 

 any rise until the base of Arenig is reached, when the ground rises 

 very steeply to the twin-peaked summit oF Arenig Fawr 

 (2800 feet), this being the most conspicuous height in the district. 

 There is a far more gradual rise towards the hilly country lying on 

 the north. The vallej'S are, as a rule, conspicuously deep and 

 steep-sided, and are occupied by swiftly running streams, though, 

 with the exception of Nant Rhyd Wen, they are still largely filled 

 with Drift. Only one stream appears to show any features of 

 interest, and that is the Hirnant: starting as a strike stream, it 

 soon makes a bend at right angles to its former course through 

 Bwlch-yr-Hwch, although the existence of a well-defined channel 



1 Geol. Mag-. 1900, pp. 204, 241. 



2 Ibid. 1892, pp. 114, &c. 



