GEOLOGY 01' CEXTKAL WALES. 



157 



common in both grits and slates. The main dip is decidedly 

 easterly. 



Mounting the higher ground above Nant y Moch the grits become 

 more conspicuous, forming a terraced structure by their outcrops 

 around the hill ; and near the summit a rugged stcplike structure is 

 produced by the regularly bedded fine-grained grit series. The 

 finest exposure of these Plynlimmon grits is in the magnificent pre- 

 cipices fronting the Eheidol lake (Llyn Llygad Eheidol), where 

 many of the beds are very thick (7-10 feet) and of moderately 

 coarse grain. A particular feature in them is their regular columnar 

 jointing; this is quite as regular as in many columnar felsites, 

 numerous columns, 7 feet long, being seen running right across the 

 beds. Conglomerates are uncommon ; but some three or four beds 

 of beautiful 1 1 pudding stone," with white quartz pebbles in a f el- 

 spathic sandstone matrix, are seen upon the western slope of the 

 mountain. 



Coarser beds of this group are probably indicated by a large 

 boulder upon the hill-top above Gogerddan House, near Aberystwyth, 

 which has pebbles as big as cannon-balls. 



These grits form a crescent zone from north of Plynlimmon 

 through the high ground of Hid Wales, running east of Tregaron 

 and east of Lampeter; but they die out beyond this to the south. 

 They constitute the genuine backbone of South Wales, lying in a' 

 synclinal. The highest beds form the upper ground of Plynlimmon 

 itself. In the Teifi area the beds are remarkable for being coarsely 

 cleaved into large tiles about ^ inch to 1 inch thick, a structure 

 which graduates into jointing on the one hand and true slaty 

 cleavage on the other. Similar cleaved grits occur around Llyn 

 Fyrddyn and on the hills east of Lampeter, all of which I would 

 place on the same horizon. There is, however, some doubt in my 

 mind as to the exact position of this horizon in the geological 

 series. 



Foldings. — As a group, the Plynlimmon grits are characterized, 

 not by violent contortions or sharp bendings with fracture, but by 

 broad and gentle foldings. These are beautifully seen around the 

 Teifi lakelets and Llynodd Ieuan ; also at Llyn Fyrddyn Each and 

 Llyn Fyrddyn Fawr, and Llyn Pugeilyn, where the ridges and minor 

 hog-backs of the surface correspond with the structural anticlinal 

 and periclinal foldings of the rocks with great regularity and 

 clearness. 



Amongst the rock-varieties we may notice blue shaly slates, blue 

 slates in 6-inch beds, cleaved into oblong flags about h inch thick 

 (Llynodd Ieuan), and arenaceous mudstone, broken up into coarse 

 rab, the fragments measuring 1-3 feet x 3 inches x 1 inch, also 

 smaller rab. 



The lithological differences between this series and the Aberyst- 

 wyth group are the greater thickness of the grit beds, the presence 

 of quartz conglomerates, and the rarity of curious rock-surfaces and 

 fucoidal markings. The slates also are, as a whole, of paler type. 



Fossils have not been discovered in the grit series of the Plyn- 



