184 ON THE PROBABLE THICKNESS OF THE OLD RED SANDSTONE. 



Probable thickness of the Old Red Sandstone. 



The maximum thickness of the strata which compose the great British system of the 

 Old Red Sandstone may best be computed by an examination of the various groups of 

 rock which rise from beneath each other, between the edge of the South Welsh coal 

 basin and the Silurian rocks of Radnorshire. In a space of about eleven miles, from the 

 outlier of the carboniferous limestone called Pen Cerrig Calch near Crickhowel (see 

 p. 163. and PI. 31. fig. 1.), to the upper beds of the Ludlow rocks in the Clyro Hills 

 west of Hay, all the intervening strata of the Old Red are conformably inclined and 

 apparently without dislocations, at gentle angles of inclination, decreasing to five or six 

 degrees as they approach the south-east boundary, and increasing to fifteen degrees as 

 they rise towards the north-west or outcrop of the older rocks. In this wide space, 

 there is no possibility of ambiguity or misconception, for the whole of the beds are suc- 

 cessively exposed in lofty escarpments, which are clearly laid bare in the sides of the 

 deep ravines by which the mountains of the Black Forest are fissured. The heights of 

 these mountains vary from 1800 to 2500 feet. 



Now provided the old notion were true, that strata preserved their dimensions on the 

 dip and passed regularly beneath each other for great distances, there would be no dif- 

 ficulty in determining, by a simple trigonometrical calculation, the thickness of the Old 

 Red System, which in this instance would be very enormous. But it is now a well as- 

 certained geological datum that no such regularity exists, and therefore an attempt to 

 deduce the vertical dimensions of formations, from their superficial breadth and the dip 

 of the beds is no longer admissible. I think, however, that looking to the altitude of 

 the mountains and the wide area they cover, the united thickness of the Old Red 

 System, at a moderate calculation, cannot be less than nine or ten thousand feet. 



