210 REV. J. r. BLAKE ON THE 



curious form not seen elsewhere, — perhaps the J. Bleicheri of 

 Boulogne. These beds have a thickness of some 8 ft. ; and they are 

 succeeded by a strong solid block 3 ft. thick, for which the quarry 

 is worked. This contains few or no Trigonice, but is full of Perna 

 Bouchardi and oysters. Its base is full of dark-coloured stones of 

 various materials and sizes. Among them are Lydian stones and 

 light-coloured quartz ; but some are hardened phosphatic nodules 

 derived from fossils of previous formations, among which a Cardium 

 and a Pleuromya are recognizable, doubtless from some part of the 

 Kimmeridge Clay. The inference to be drawn from these nodules 

 must not be overlooked. Glauconitic grains, derived doubtless in 

 the same manner, are usually characteristic of the sand below ; and 

 similar nodules occur in the lower beds at Tisbury ; but all are absent 

 in the district of Portland. Hence these more northern districts 

 were more rapidly upheaved than those to the south, and brought 

 earlier into the conditions which are necessary for calcareous 

 deposits. Below this hardened block comes a mass of argillaceous 

 shell-conglomerate, like No. 5 of Swindon, to be noticed hereafter. 

 All these features indicate that the fossiliferous Bourton beds, though 

 of a similar structure to the TW^oma-beds at Swindon, represent 

 a slightly earlier date as well as that at which the latter were 

 formed. These, therefore, are the earliest rocks which in this 

 district can be referred to the Portland Stone, and are parallel to 

 the Trigonia-hed at the base of the Tisbury freestone. 



The similarity of the succession noticed in the various districts as 

 yet described shows that the order of events was the same, but aifords 

 no proof that they were synchronous ; the evidence of the fossils, on 

 the contrary, goes to show that the more northern were the earlier ; 

 and this we must bear in mind in interpreting the higher beds. 



The " basal sands " of Swindon have been seen to be covered with 

 shelly masses abounding in Astarte rugosa, just as the freestone of 

 Chicksgrove is. These masses have other fossils in common ; but 

 the GeritJiiurn is not concavum, but portlandicum, and therefore 

 ceases to be a guide, and we must take the base of the chalky beds 

 of Chicksgrove and Fpway as the anterior limit for the age of No. 1 

 here. If the Purbeck be always of the same age, the posterior limit 

 would be the age of the Eoach ; and thus the few small beds, limited 

 to the N.E. of the Swindon quarry, would represent a long interval 

 of time — too long for their obviously rapid deposition. No doubt 

 there may have been Ceriilimm-'hQd% overlying / (if that be truly 

 Portland), of which the blocks in g are the only remnant ; but there 

 is no proof of this. Moreover we must not forget the difference 

 in character between the Purbecks of Swindon and those of more 

 southern districts. In the latter we have uniform deposits over con- 

 siderable distances, lying on unevenly but not deeply eroded Portland 

 rocks ; in the former there are the carvings of rivers, the transported 

 blocks, and the rapid dying-out of deposits — all features charac- 

 teristic of subaerial action. Seeing, then, that we have reason to 

 believe that the earliest " Portland Stone " here antedated that at 

 Portland, we are justified in concluding that the land here emerged 



