454 SILURIAN SYSTEM — LUDLOW ROCK. 



together, as along the western flank of the Tortworth tract; for while we. trace the 

 persistence of the Old Red Sandstone from Berkeley to the south, and confidently 

 assert that it forms the substratum of that portion of the district, we cannot definitely 

 mark its boundary, though we know that the red and green marls of Whitecliff Park 

 and Sunday Hill, from their conformable passage into the lias, must belong to the New 

 Red System. Whenever the sandstone of the older formation appears, or that the face 

 of the rock is highly inclined, (a feature not observed in the New Red of this tract,) 

 the difficulty vanishes, but unluckily such good evidences are exceedingly rare, and the 

 Old Red Sandstone is in many situations quite as little inclined as any of the overlying 

 formations. At Thornbury and at Kington the conglomerates, hard sandstone and 

 grits of the Old Red dip only 5°, 10° and 12°. The day, indeed, has now passed by, 

 for estimating the age of sedimentary deposits by the angle of inclination in which 

 their strata are placed, since we know that in disturbed regions the cretaceous and even 

 the tertiary strata are often vertical. 



Silurian System. 



Though occupying a considerable area, the rocks composing the Silurian System are 

 clearly exhibited at only a very few points. Over a large portion of this area, their 

 exact places are not easily assigned, owing to their small development and the great 

 extent to which they have been denuded, and it was not till a fourth visit, that I was 

 able to determine their precise extent and range. One of the chief difficulties, already 

 alluded to, lies in the red colour which prevails over two thirds of the district, whether 

 composed of the New Red, Carboniferous, Old Red, or Silurian Systems. 



Ludlow Rock. 



The best representatives of the Ludlow rock which I have detected are the beds seen at low 

 water on the west bank of the Severn, at Purton Passage, where rising in a dome-like form, and 

 dipping beneath the Old Red Sandstone of the opposite bank of the river, they are partially over- 

 laid by that rock at the Ferry-house. (See wood-cut, p. 446.) The section (PI. 36. f. 17-) also 

 represents the succession of the Silurian strata at their point of emergence on the Severn, where 

 they dip away on three sides at angles of 15° to 20°. The uppermost beds are somewhat obscured 

 by mud, and the detritus already alluded to as having entirely covered the adjacent fossil beds of 

 the lias. These upper beds or Ludlow rocks are of dark greenish grey colour, somewhat micaceous, 

 and contain Leptama lata, Orthis unguis, &c. Beneath them is a very thin course of impure lime- 

 stone, containing Terebratula JVilsoni, and representing the Aymestry limestone. Other beds 

 of a mudstone character succeed, with occasionally flattened spheroidal concretions ; and these 

 strata, the equivalent of the Lower Ludlow, pass down into shelly calcareous beds, representing 

 the Wenlock limestone. This succession is laid bare on the shelving shore, between low and high 

 water marks, in a number of thin-bedded ledges which wrap over each other, the highest stratum 



