Geological Society. 2'29 



tite, &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 grauwacke rocks, have this year been extended west- 

 ward to the source of theTeme, and 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 of the mineral structure of all the groups of the 

 formation afforded by many natural deep sections, is insisted upon 

 as demonstrating the hopelessness of ever finding coal in the old red 

 sandstone of this part of the kingdom. 



The maximum thickness of the whole formation is estimated to be 

 from 10 to 12,000 feet. 



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

 Old Red Sandstone. 



A very remarkable outlier of carboniferous limestone and millstone 

 grit is pointed out as occupying the summit of a mountain of old red 

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

 Pen Cerrig Calch, is distant from the main escarpment of carbonife- 

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

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

 inclination of the beds, that the limestone of Pen Cerrig Calch 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 complicated dislocations of great extent occur in that 

 segment of the extensive margin of the South Welsh coal-basin which 

 extends from the Caermarthen Fan to the latitude of Llandello. 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 re- 

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

 at the spot marked by the occurrence of the glazed limestone ; but 

 the most extraordinary of all these disruptions is that which has 

 given rise to the position of the singular outlier ot carboniferous lime- 

 stone called Castel Cerrig Cennen. This outlier, by a violent ele- 

 vation of the old red sandstone, has been dismembered from its pa- 

 rent rock, and left insulated, 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- 

 boniferous limestone are (as it were,) thrown out en echellon from 

 the circumference of the coal-field into the area of the old red sand- 

 stone. 



This portion of the memoir concluded with a comparison between 

 the local and violent disturbances which have broken up the moun- 

 tain limestone and old red sandstone on the margin of the coal-licld, 

 and that much greater movement from north-east to south-west which 

 threw up, in Brecknockshire and Cacrinarthenshirc, the mural chain 

 formed of the lower member of the old red sandstone, and the upper 



