316 GEOLOGY OF THE BBISTOL COAL-FIELD. 



the Devonian sandstone has been forced through a large thickness 

 of limestone, supporting the latter on both sides like the roof of a 

 house, and forming what the geologists call an anticlinal. After 

 this period this peculiar form of upheaval seems to have ceased. 

 All the later faults, though numerous, are mere cracks as it were in 

 the strata, interrupting the continuation of strata only. 



The Carboniferous system is divided into two grand divisions — 

 the Limestone rock proper, and the Coal measures. The first and 

 oldest of these is sub-divided into three groups, viz. — 



The lower shales about 500 feet in thickness. 



The mountain limestone about 2000 do. 



The upper shales about 400 do. 



In comparatively recent times, the limestone beds became worn 

 down by the action of air and water. A familiar instance of the 

 effects of the long continued wave action must be familiar to all who 

 visit "Weston-super-mare. When standing on the summit of Brean 

 Down and looking towards the Steep Holmes at dead low water, the 

 ripples on the water distinctly mark the edges- of the limestone 

 strata that are only just covered by the sea. These prove that the 

 island rock was once connected with the shore, but now separated 

 by denudation. Their position is explained by fig. 17. 



Fig. 17. — Section hetioeen Wedon and Steep Holmes. 



Brean Down. Steep 



High Water Mark. Holmes. 



The magnificent and continuous section oithe first division of the 

 Carboniferous system is so perfectly exposed on the Gloucestershire 

 side of the Avon, that we will take it as a fine summary of the 

 whole series as observed in the counties of Gloucestershire and 

 Somersetshire. The junction of the Devonian into the Carboniferous 

 strata is complete, and the [gradual passage into the lower shales 



