204 REV. J. F. BLAKE ON" THE 



side of the great quarry at Swindon." It is obvious that a quarry 

 that can give rise to such ideas must show a very remarkable 

 section ; and in fact a more complicated and at the same time 

 instructive series of sections I have never seen. The whole result 

 of their study, however, is, distinctly to negative the idea of any 

 intermixture of formations, and to show that the one is unconform- 

 able to the other. 



I have carefully traced every bed through every part of the 

 quarry, and have examined each for fossils, so far as to settle the 

 question of the marine or fluviatile origin of the beds ; and I now 

 present a continuous section (figs. 2-7, p. 205) which will, I believe, 

 demonstrate the truth of the idea of unconform ability. 



The particular section which really shows a very peculiar 

 development of true Portland rocks beneath the Purbeck is in the 

 N.E. corner (fig. 2) ; it will therefore be best to begin at that point 

 and trace the beds either way. 



The northern face of this quarry shows an eroded surface of the 

 main Portland rocks of the district, which for the sake of distinc- 

 tion may be called for the present the basal sands. In the hol- 

 lows of this surface and for a few feet above are some fiaggy lime- 

 stones, the relations of which to the deposits succeeding to the 

 south are obscured by a " pot-hole," but which appear to belong 

 to a set which overlie all the other beds in these quarries. No 

 fossils have been discovered in these flags. The eastern face shows 

 also an eroded surface, but a more regular one ; and lying on it is 

 (a) a lenticular mass of brownish earthy sand ; it is of very limited 

 extent, as it may be traced in less than a hundred yards from its 

 commencement to its close, and is never more than 4 feet thick. It 

 contains black carbonaceous patches ; and hardened parts of it have 

 Trigonia gibbosa, Cerithium jportlandicum, and Lucina in them. A 

 tooth.oi Goniopliolis and freshwater shells have been reported from here. 

 Upon this is (6) a white creamy limestone of more uniform thickness, 

 but of no greater horizontal extension. It has an even surface below, 

 but above is festooned by the encroachment of the succeeding beds 

 before its consolidation. This contains Tiirritella (cf. minuta^ Koch 

 & Dunker), and a little Gasteropod like a smooth Turbo, but which 

 has not sufficient character for naming. In some places it also 

 contains abundance of Cerithium portlandicum. The next deposit 

 (c) is a dark earthy clay, at first sight like the material of a " dirt 

 bed ;" but both this and a are distinguished from the somewhat 

 similar overlying beds by the absence of any rolled stones. The 

 dark clay is no more extraordinary than similar deposits in the 

 Chalk. The termination of these three takes place together at the 

 same spot, where they are all rounded off between the succeeding 

 deposit and the basal sands, as in the figure. Overlying this, and 

 forcing it to take up its irregular shape, is (d) a shell limestone 

 composed of Cerithium portlandicum and Astarte rugosa, with more 

 occasionally Trigonia gibbosa, Pecten lamellosus, Cardium dissimile, 

 Neriioma sinuosa, Buccinum angulatum, and Corbicella Morceana. 

 These have collected in hardened blocks, which sank into the soft 



