258 



THE CARBONIFEROUS SYSTEM. 



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both sides of the estuary of the Avon, not directly in contact with 

 the shales, etc., which intervene between them and the metamorphic 

 hills, but in such positions as to leave no doubt 

 | as to their relative age. One of the best expos- 

 | ures of these rocks in this vicinity is on the right 

 n if bank of the Avoir, immediately above the Windsor 

 = bridge, and I shall describe this section in detail, 

 that the reader may at the outset be familiarized 

 with the principal members of that great gypsi- 

 ferous series which occupies the greater part of the 

 district now under consideration. 



The first rock seen south of the bridge is a thick 

 I s bed of red marly sandstone, a soft rock coloured 

 red by peroxide of iron and cemented by carbon- 

 ate of lime. Below this is a bed of greenish 

 marl, similar to that above in composition, but 

 wanting its colouring matter. Then there is a 

 thick gray limestone, containing enough of frag- 

 ments of shells to lead us to infer that it may 

 have been made up of such materials, but so 

 decomposed and agglutinated together that it 

 appears now a compact, almost non-fossiliferous, 

 rock. Below this we find again red and greenish 

 marly sandstones. The whole of these beds dip 

 to the north at an angle of 50°. At this point, 

 however, there is a fault, marked by a little gully, 

 cut in consequence of the surface water finding 

 a more ready passage at this place. The next 

 beds seen are again red marls, but dipping to the 

 south at an angle of 55°. On these rests a 

 yellowish limestone, above which are more red 

 and greenish marls.* Next we have another lime- 

 stone of flaggy or laminated structure, with a 

 number of fossil shells scattered over some of 

 the surfaces, as if they had lived on these surfaces 

 or been scattered over them after death. These 

 shells, like those of the Cumberland Lower Carbon- 

 iferous limestones, belong to the genera Productus, 

 Spirifer, and Terebratula, all shells of the same 

 family, one of the most singular of the tribes of 

 bivalve shell-fish, and, in so far as we can judge 



* According to Mr Hartt, this bed also dips to the N.W., and the break occurs 

 immediately to the south of it. 



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