450 



NEW RED SANDSTONE NEAR TORTWORTH. 



mounting the green and red marls of the New Red Sandstone. In an insulated fork of lias, a prong 

 of which advances to the south of Charfield Church, I found slabs of this bed highly charged with 

 Saurian bones, coprolites, &c; and Dr. Cooke informs me that he had previously collected similar 

 remains in the lias overlying the red marl under Wotton. 



The great mass of lias in the Vale of Gloucester is in general so flat, and so much covered by the 

 fine debris of the oolite escarpment, that any estimate of the destruction it has undergone, can be 

 formed only by considering the depth to which the vale has been denuded ; but when we examine 

 the sides of the broken ground of older rocks, of which the Tortworth district is composed, we ob- 

 tain some measure of the amount of this destruction, by finding small patches of the lias adhering 

 at different levels, in one place to the carboniferous limestone, in another to the dolomitic conglo- 

 merate, frequently in patches so minute that it is barely possible to lay them down even upon the 

 sheets of the Ordnance Survey. Three of these alone are indicated in the annexed map 1 . 



: 



New Red Sandstone. 



The upper and central members of the system of New Red Sandstone, so largely expanded in 

 other parts of the kingdom, has been correctly defined in this neighbourhood by Mr. Weaver, as 

 consisting chiefly of red clay and marl, and as being specially distinguished by the presence of stron- 

 tian, a mineral which the same author has pointed out as occurring also in rocks of different ages 

 in the Tortworth district. As the passage of the red marl into the lias is here well exposed, so 

 also on the sides of the road east of Wickwar is a perfect transition in descending order of these 

 marly beds into beds of sandstone, and thence into beds of dolomitic conglomerate. 



" Magnesian Limestone" = " Dolomitic Conglomerate*." 

 The rocks forming the base of the New Red Sandstone in the south-west of England have been 



1 Dr. Cooke pointed out to me other small patches of lias near Tortworth. For an acquaintance with one of 

 them near Thornbury, I am indebted to Mr. Fry, of the Round Tower on Olveston Down. 



Owing to one of the most modern changes of the surface, a well known locality of lias is no longer visible, and 

 collectors can never more repair to the shore at Purton Passage to gather those lias fossils which ornament so many 

 museums. The process of encouraging the mud of the Severn to accumulate upon lines of pile and osier, has 

 been so effectually practised by Lord Segrave, that the fossil beds which appeared as ledges at each ebb tide, have 

 been buried under slimy sediment, which by an additional embankment will at no very distant day pass under 

 the plough of the husbandman. It was with infinite delight, therefore, that I found this mud had not yet 

 extended so far as to obscure the instructive arch of Silurian Rocks, which is still partially seen at low water 

 rising from beneath the overlying deposits and forms the foreground of the vignette, p. 446. 



2 The term "Dolomitic Conglomerate" was first applied to this rock by Messrs. Buckland and Conybeare, 

 at the suggestion of Mr. Warburton, and on the authority of Von Buch. " That definite triple salt, the car- 

 bonate of lime and magnesia, when found native in a state of purity and associated with primitive (or altered) 

 rocks, has received the name of dolomite ; and it is the same salt, intimately but mechanically blended with 

 iron or bitumen, or carbonate of lime, which in beds forming part of our New Red System have been called 

 magnesian limestone. It appears to us desirable to use but one appellation for substances not essentially dif- 

 fering in chemical constitution." — Geol. Trans., N. S., vol. i. p. 212 note. 



