GEOLOGY OF CENTRAL WALES. 



153 



jointed," and " pale slates with ribbon bandings." The prevailing 

 colour of the rocks is a pale blue-grey ; and they generally break up 

 into moderately small fragments, shale-fashion. 



Slate. — In many places where the slates are better cleaved, and 

 less frequently jointed, they are worked in small quarries for inferior 

 local purposes, as around Llancynfelin, in the hills south of Machyn- 

 lleth, in the island mounds of the Gorsfochno flats, and near the 

 Devil's Bridge. These rough slates are pale-coloured, hard, and 

 coarse : but around Machynlleth thoroughly good slates are worked 

 in a number of large quarries. One of the largest of these is at Pont 

 Faen, where large slate-flags are extracted ; but the most interesting 

 to the geologist is that at Morben, two and a half miles south-west 

 of the town of Machynlleth. These are thick-bedded slate rocks, 

 with a few thin grits, dipping at high angles to the W.K.W. 



This, however, is an inverted dip, as is proved by the position of 

 the prominences on the grit-bands, which are upon the upper sur- 

 faces of the grits, the worm-trails being also manifestly in inverted 

 position. The cleavage-dip diverges only slightly from the bedding- 

 dip, so that the Graptolites usually run for some distance along the 

 surface of the slate, and then gradually skim under it. The slates 

 are very good, dark and soft. Nodules of iron-pyrites occur; and 

 nearly all the Graptolites are converted into this bright mineral. 

 This quarry has yielded the finest of our fossils, the Monograptus 

 SedgeivicJcii being especially magnificent. 



Fossils from Morben Quarry, Machynlleth. 



Rastrites peregrinus, Barr. Monograptus Hisingeri, Carr. 



■ inaxiinus, Carr. tenuis, Portl. 



Monograptus Sedgewickii, Portl. j involutus, Lapw. 



-, var. spinigerus, Nich. 



, var. distans, Portl. 



intermedins, Carr. 

 spiralis, Geinitz. 

 lobiferus, M'Coy. 



Diplograpsus Hughesii, Nich. 

 Climacograpsus normalis, Lapiv. ? 

 Myrianites M'Leayi, Murch. ? 



Lapworthii, Keep. 



Buthotrepliis major, Keep. 



Below this in present position, but by inversion really above it, is 

 a zone of pale blue and olive shaly slate, pointed out to me by 

 Mr. J. E. Marr, F.G.S., as being similar to certain zones in the 

 Coniston mudstones. The great slate district of Corris, usually 

 referred to the Llandeilo age, is in this series, as will be shown 

 later on ; also the excellent slates of Dol y Mynach and Cwm par 

 Adwys, west of Bhyader. 



Other rock-varieties, such as dark paper-shales, rab, and mud- 

 stone, approach near to the Aberystwyth types ; and some of the 

 occurrences of such rocks doubtless indicate the uprising of that group 

 as inliers in the metalliferous-slate area. In the .Devil's-Bridge 

 road such rocks appear in several places. 



Grit-beds as thick as 6 inches may occur ; and thin zones, about 

 J inch to 2 inches thick, are frequent; these are generally red- 

 stained at the surface, and micaceous. They are trustworthy guides 

 to the dip of the beds, which in their absence is often not easy to 

 discover. 



