ASSOCIATED EOCKS OF THE BREIDDEN HILLS. 533 



the difficulties which beset me in trying to make anything out of the 

 petrology of rocks which have undergone so much decomposition as 

 these. 



III. Cambria:n^ Rocks (Sedgwick). 



These are best exposed in a little brook which rises between 

 Bulthey and Bausley Hills, and, joining Belleisle Brook, cuts through 

 the shales where they are not much traversed by eruptive rocks ; but 

 they are also seen in many isolated spots amongst the hills, particu- 

 larly -near Trewern. They consist of a series of slightly varying 

 micaceous shales, dark grey and sometimes black in colour, easily 

 fissile, slightly concretionary, and much broken by ironstained joints. 

 Though appearing very likely to be fossiliferous, no fossils of any- 

 kind have been found in these Criggion shales, and their rerharkable 

 homogeneity renders it useless to attempt to establish divisions 

 amongst them. The base of the shales is not seen, for the Severn 

 alluvium covers it up. A remarkable band of sharply jointed, struc- 

 tureless, black quartzose grit occurs about 800 feet above the lowest 

 beds seen ; it is 20 feet thick, and is evidently much altered by a 

 small dyke of diabase which penetrates the shales in its neighbour- 

 hood, and by its proximity to the intrusive mass of Brimford Wood, 

 of which this dyke is an offshoot. There are two other dykes, one 

 60 feet and the other 30 feet wide, shown in the tributary. Along 

 Belleisle Brook which follows the junction of the shales with coarse- 

 grained diabase, the shales are much hardened, jointed, and altered. 

 The dip of the rocks along the brooks varies from 80° S. to 60° S.E. 

 and 60° S.S.E. ; and, from the map, I should calculate that, below the 

 conglomerates shortly to be described, there are about 2700 feet of 

 rock where least disturbed, though the changes of dip and amount 

 of intrusion render this estimate uncertain. Shales of precisely this 

 character are observed to the S.W., often more or less altered and 

 much disturbed in dip (particularly in the neighbourhood of the 

 Criggan, Garreg, and other intrusive masses), and always uufossili- 

 ferous. I have not been able to identify the black grit to the south 

 of this section. 



The shales are followed by volcanic ashes, conglomerates, and 

 even lavas which rise in the hills of the S.E. ridge. They are typi- 

 cally developed in the road-cutting between Bulthey and Bausley hills 

 and on the northern crag of Bausley Hill where the rock is almost 

 vertical. In a quarry near the road-cutting, spotted ashy beds and 

 conglomerates are exposed, and these are followed b}' a massive con- 

 glomerate (of the road-cutting) dipping S.W. exclusively com- 

 posed of volcanic rocks, and almost entirely of andesitic fragments, 

 some of them reaching a diameter of 18 inches, set in a grey or 

 spotted ashy matrix. The thickness of conglomerate shown in the 

 road-cutting is 110 feet. In Bausley Hill* shales and ashy grits 

 are intercalated amongst the conglomerates, and consequently the 

 series is thicker ; a few fossils are found in the grits. In a small 

 farmyard at the north end of Bausley HiU, above the conglomerates, 

 * Sil. Syst., 292. 



