12 



Remains of Crustacea have been found in thisgroup, together with 

 defences of fishes, &c. 



c. The tile-stones are best exhibited in a remarkably rectilinear es- 

 carpment, extending from the north-western extremity of the Mynidd 

 Eppint to near the mouth of the Towey, a distance of about thirty- 

 five miles. 



These beds contain fossils in Caermarthenshire, and also in their 

 north-eastern prolongation into Shropshire : among them are Lingula, 

 Avicula, three or four species of univalves, a small species of Orthoce- 

 ras, &c. These fossiliferous tile-stones constitute the beds of passage 

 into the " Ludlow Rock," or highest member of the grauwacke series. 



The limits of certain detached basins of the old red sandstone, par- 

 tially described during the last session, and which are spread over the 

 area of the inferior Ludlow rocks, have this year been extended west- 

 ward to the source of the Teme, twenty-five miles to the north-west 

 of the ancient line of demarcation. The absence of all vegetable re- 

 mains, with the exception of a few small fragments, notwithstanding 

 the full exhibition afforded by many natural, deep sections of the 

 mineral structure of all the groups of the formation, is insisted upon 

 as demonstrating the hopelessness of ever finding any workable quan- 

 tity of coal in the old red sandstone of this part of the kingdom. 



The maximum thickness of the whole formation is estimated to be 

 about 10,000 feet. 



II. Outliers of Carboniferous Limestone, &c. ; Dislocations of the 

 Old Red Sandstone. 



A very remarkable outlier of carboniferous limestone and millstone 

 grit, is first described, occupying the summit of a mountain of old red 

 sandstone to the south of the town of Crickhowell. This mass, called 

 Pen Cerrig Caleb, is distant from the main escarpment of carboni- 

 ferous limestone from four to five miles, and is separated from it by 

 the deep valley of the Usk. It is shown, by the position and slight 

 inclination of the beds, that the limestone of Pen Cerrig Caleb must 

 have been connected with that of the main escarpment anterior to 

 the excavation of the intermediate valley, and the case is cited as one 

 of the deepest and most extensive denudations which has come within 

 the author's observation. 



Numerous and complicated dislocations of great extent, occur in that 

 segment of the margin of the South Welsh coal-basin, which extends 

 from the Caermarthen Fan to the latitude of Llandeilo. The largest 

 of these breaks is the great upcast of Fan Sirgaer, by which the old 

 red conglomerate is thrown up about 700 feet from its regular 

 horizon at Cerrig Ogof. The greatest downcast has taken place at 

 the spot marked by the polished limestone ; but the most extraordi- 

 nary of all these disruptions is that which has given rise to the po- 

 sition of the singular outlier of carboniferous limestone called Castel 

 Cerrig Cennen. This outlier, by a violent elevation of the old red 

 sandstone, has been dismembered from its parent rock, and left in- 

 sulated with the dip of its beds reversed, in the centre of a valley of 

 the old red sandstone. 



By these great elevations and subsidences large masses of car- 



