548 Murchison's Silurian System. 



associated with reddish shale, and the decomposition of the 

 mass uniformly produces a red soil, by which character alone 

 the outline of the division is easily defined, being always 

 clearly separable from the upper beds of the Silurian sys- 

 tem, which decompose into a grey surface. 



In Shropshire and the contiguous parts of Herefordshire, 

 this lower member of the old red sandstone rarely occupies 

 high ground, and being for the most part recumbent on the 

 talus of the upper Silurian rocks, where the latter sink 

 down into vallies, it is generally much obscured by detritus. 

 In a gorge near Ludlow, flag-like micaceous dark red sand- 

 stone, " bur-stones" of the peasantry, rise at an angle of about 

 fifteen degrees from beneath the red argillaceous marls 

 of Oakley park, and pass down into a light coloured grey, 

 yellowish, and greenish grey freestone, which will presently 

 be described as constituting the upper stratum of the Silu- 

 rian system. 



The " bur-stones" are here seldom so fissile as the " tile- 

 stones." They occasionally contain a few organic remains, 

 such as Avicula, and a small Lingula, which will presently 

 be described ; but in some parts the remains of fishes prevail 

 more than Mollusca, particularly Dipterus macrolepidotus, 

 Sedg. et Murch., and others of the genus Onchus, Ag., palates 

 of fishes and bufonites.* In the southern parts of Car- 

 marthenshire and in Pembrokeshire, the tilestone cannot be 

 traced as a persistent zone, and the triple subdivision of the 

 system can no longer be observed, but the previous sketch 

 of the lower member of the old red sandstone Mr. Murchison 

 derived from numberless transverse sections along an in- 



* We shall probably give examples of the fossils illustrative of the 

 important distinctions here pointed out by Mr. Murchison. We are only 

 anxious before entering upon this part of the subject, to be favoured 

 with the examination of any organic remains that may have been found 

 in ancient beds in India, which there may be reason to suppose equi- 

 valent to the rocks described by Mr. Murchison. 



