Chalk Strata of Ballard Head. 4 1 5 



them the transverse crack through which the Stour flows was 

 formed. At Corfe Castle, the strain which raised Purbeck 

 has also cracked the chalk in two places; and it is on the 

 portion left between them that the castle is built ; two small 

 streams finding their way from Purbeck to Poole harbour, in 

 the same way as the Stour flows through the chalk at Bland- 

 ford. There are other indications of convulsion in the 

 chalk, as exhibited in the pits at Sturminster, Lytchett, 

 Hinton, Witchampton, &c. At the latter place, it is frac- 

 tured into a variety of small angular pieces, and so completely 

 saturated with ferruginous matter, that it partakes of the 

 character of a soft yellow marl, as it does also near the 

 junction of the chalk and plastic clay in Studland Bay. 



From the presence of a bed of rolled siliceous pebbles, ex- 

 tending from one end of the Purbeck range to the other, and 

 from the occurrence of the same at Hinton Martell, where 

 there is a considerable accumulation of them, it would seem 

 that the intermediate space formerly formed the bed of a 

 gulf in the tertiary sea, the beach of which is indicated by 

 these pebbles. It is probable that the summit of Badbury, 

 and some other points of chalk, formed islands in that gulf; 

 and these, together with the bottom of it, and all the accumu- 

 lated deposits of sand and clay, &c, now forming the present 

 surface of the plastic clay series, were elevated en masse ; 

 during which occurrence some portions of the chalk were 

 heaved higher than others, lifting up and throwing down the 

 superjacent tertiary beds on either side of certain lines, 

 which are parallel to Purbeck, and which indicate, no less 

 than the phenomena of the drainage of the whole country 

 between Poole harbour and the river Avon (which enters 

 the sea at Christen urch harbour, where the London clay 

 deposit commences), that these minor derangements were 

 produced by similar forces, acting similarly to those to which 

 we are indebted for the interesting circumstances disclosed by 

 the Purbeck range. 



The line of strain appears to be, in all these cases, from east 

 to west ; and this strain, so acting, has necessarily induced, 

 either, in its own longitudinal direction, a series of longitudi- 

 nal fissures, such as those in which Poole harbour, Christ- 

 church harbour, &c, have been excavated, and the feeding 

 waters of drainage flow ; or transverse cracks, of which two 

 examples have been quoted at Corfe Castle and Blandford, 

 and of which others may be found in the terminal sections 

 forming the cliffs at Ballard Head, at the Needles, Culver 

 Cliff, &c, and in the cracks in the Purbeck chalk at Three- 



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