508 EEV. J. p. BLAKE OIT THE 



reous and quartzose bands, may be traced in the valleys north of 

 Coed Mawr, where the rocks are beautifully displayed. 



But the most important feature is the occurrence of the sporadic 

 limestones and quartz-knobs. Of the former we have several 

 examples. There are two on the road between Garth Perry and 

 Beaumaris ; of these the north ern one, at the letters " is " of Tyddyn- 

 isaf, stands out into the road and is limited to a horizontal breadth 

 of about 10 yards. It is broken by joints, and contains a purple 

 slate band, which is torn into tongues and shreds in its midst and 

 includes within it fragments of the limestone, which is compact and 

 reddish in tint, like that at Cerrig Ceinwen. Had this rock been 

 composed of the materials of an igneous rock, we should have had no 

 hesitation in calling it intrusive. As it is, we may regard it perhaps 

 as two adjacent lenticles, themselves, it may be, precipitated from cal- 

 careous springs, and which have subsequently been bent and broken, 

 and carried the slate with them. It is not often that the actual 

 relations of the limestone to the surrounding rocks can be seen so 

 well as here ; but even then we are not entirely resolved as to its 

 origin. There is another limestone-knob of somewhat larger size, 

 which is quarried near the upper road at Tyddyn-isaf itself ; there 

 are some which almost amount to pure limestones in the valley below 

 Bryn-cogel, north of Beaumaris, and at a quarry now worked out 

 where the limestone used to be burnt at Ehyd Eilian, west of Llan- 

 degfan. This limestone is mosaic in structure, and most like that 

 near the Druid Inn. 



The only quartz-knobs I have seen are one in Cadnant Yale, 

 which is more like a reef, and that at Pen-y-parc described by Prof. 

 Bonney. I certainly regard this latter as having the same origin as 

 the other quartz -knobs, on account of the mode in which it occurs. 

 The view of the north side of the pit, which has been worked for the 

 quartz, is correctly described by the words " the upper surface is 

 rather uneven ; " but the appearances of the south and east sides are 

 not adequately represented by this phrase. The eastern face is 

 represented in fig. 16. Here the structural lines of the schist are 



rig. 16. — Eastern face of Quartz-hioh, Pen-y-Parc, Beaumaris. 





1 2 



1. Quartz. 2. Schist. 



vertical between the two branches of the knob, and curve round it 

 on the southern side, where they also contain some lenticles of 

 quartz. The southern face also shows the lines of the schist risino- 



