GEOLOGY OF THE BRISTOL COAL-FIELD. 



263 



of Wales, Scotland and Ireland. When these had reached the 

 depth of many thousand feet, and the bed of the ocean had become 

 suitable for the lowest sea weeds, and animals of the Molluscan, 

 Annelidan and Crustacean kind, the muddy strata of the Lower 

 Silurian period appeared. As, however, we do not find any of 

 these are exposed to view within the limits of our district, we 

 must pass them and examine the Upper Silurian deposits, which 

 must be taken as the earliest beds, and base of our exploration in 

 the Bristol Coalfield. 



We are fortunately able to examine these Silurian beds in the 

 neighbourhood of Tortworth and Palfield, where they are brought 

 to the surface within our reach, and afterwards well exposed for 

 study by denudation. These strata are a continuation of the 

 rocks of Mayhill, Huntley, and Woolhope. At Flaxley the 

 Silui'ian beds dip beneath the [Devonian rocks, and re-appear at 

 the eastern bank of the Severn at Purton, where they may be 

 seen as an anticlinal. 



The Silurian deposits within the boundaries of our map are 

 divided into three groups, — the Upper Llandovery or Mayhiil 

 Sandstone, the Wenlock Limestone and Shale, and the Ludlow 

 Shales. 



I. The Tl^^er Llandovery Sandstone. 



At Purton these beds are too deeply seated for observation; 

 there the upper groups only appear, and are exposed at the lowest 

 tides, as seen in the following section. 



Pig. 4, — Section near Purton. 



h c 



d 



a Devonian. — b Ludlow. — c Wenlock Limestone.— ^Wenlock Shales. — 

 e Trias.— /Rhae tic. — g Lower Lias. 



