Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 561 



marks appear to range N.N.E. and S.S.W., as if the stream had come from the W.N.W. I 

 examined the stones in a fence by the road-side, and found that nine slabs which bore clear 

 ripple marks on both sides, had the two sets parallel to each other, tending to shew that 

 the marks were due to the permanent action of a current, not to the set of the wind. These 

 slabs rest on a bed of brown sand. 



The freshwater beds which succeed next in order, and contain so many remains of turtles 

 and fish at Durlstone bay, are comparatively unimportant at Ridgway, and their chief organic 

 remains are Cyclades and Cyprides. There is, however, a thin band at the top of No. 73 of the 

 Ridgway section, which contains similar fossils to the chert beds below. The "cinder" is 

 very thin at Osmington, but swells out again at Ridgway into two beds, the upper of which is 

 composed of oysters, and the lower contains many Cardia, but I have found in it no traces of 

 the Hemicidaris Purbeckensis. 



There is a layer of fibrous carbonate of lime beneath the cinder bed, which displays upon 

 its lower surface the impressions of cracks in the clay beneath, evidently due to a desiccation of 

 the mud before the cinder was formed. This shews that the bed of the ancient river was 

 laid dry, and then deeply depressed again, not under fresh water as before, but now beneath 

 the sea, while the occurrence of the same species of freshwater shells above the cinder as 

 below, seems to point out that some portions of the area occupied by these deposits was 

 not only not subject to this subaerial condition, but also was free from that incursion of the 

 sea-water which produced the cinder bed. 



The cherty bands become more siliceous as we proceed westward, and in the same 

 proportion their fossils are better preserved. At Lul worth we have a thick bed of hard 

 white cherty stone, with a few Valvatas and other shells, but at Osmington, beneath the chief 

 bed of chert, there occurs a layer of tabular flints, resting on carbonaceous earth, whose under 

 surface is covered with shells in a fine state of preservation. At Ridgway, in the same 

 position, some very beautiful specimens may be procured, with Physa, Paludina (two species), 

 Planorbis, Valvata, turtle-bones, fish-teeth, multitudes of Chara seeds, &c. The layer of 

 purplish dirt which at Ridgway forms the upper portion of these bands, contains an admixture 

 of brackish shells, Ostrea, Avicula, Modiola ; nodules of black flint occur in it which present 

 silicified Chara stems (?) upon their surface. 



Next we come upon a band of shaly stone, ten inches thick, chiefly composed of Cyprides, 

 but full of carbonized impressions of a branching aquatic plant, which must have lived upon 

 the spot. I have placed a fine specimen in the Museum of Practical Geology in Jermyn 

 Street. My impression is, that this is the equivalent of the Zostera-band mentioned by 

 Professor Forbes in his paper, as separating the middle and lower Purbecks at Mupes bay. 



The last member of these middle Purbecks, as developed at Ridgway, is a band of hard 

 stone, eight inches thick, almost composed of Cyprides, and containing a small univalve 

 (probably Hydrobia). It contains the Cypris fasciculata (Forbes), which is characteristic of 

 the middle division of the Purbecks. 



The following is a comparative list of the principal divisions of the Middle Purbecks, 

 as developed at the extreme points of our district. 



72—2 



