Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 565 



well-known Portland dirt-bed is there also very distinguishable, but is not carbonaceous, and 

 I could perceive no trees or Cycadeae. The freshwater strata beneath it are thin — not 

 exceeding about a foot. There is here and at Worbarrow a singular bed of dark slate- 

 coloured Cypris-shale, very thin, and contrasting remarkably with the grey limestones of this 

 part of the series. 



We will now again pass to the western extremity of the district, and examine the lower 

 division of the Purbecks at Ridgway-hill. The characteristic difference of their development 

 from that at Durlstone bay consists in their being here much more calcareous and light- 

 coloured. They also afford many beds of good workable limestone ; and the dark shaly clays 

 which constitute the chief thickness of the lower division of the series at Durlstone are entirely 

 absent. After about three feet of marly limestones with clay partings, we come upon the 

 insect beds. These are much harder and thinner than at Durlstone bay, and contain a much 

 smaller variety of species*. In some spots, however, the individual elytra are fully as abundant 

 as at Durlstone. A prevailing parallelism of the wing-cases, which lie N.N.W. and S. S.E., 

 indicates a particular direction in the current. In a specimen in which there are two layers 

 of elytra, about an inch and a half apart, the direction is the same. The direction indicated by 

 these remains is therefore 45° more to the north than that indicated by the ripple-marked 

 sandstone of the middle Purbecks. At Worbarrow there are two beds with ripple marks, 

 nearly at the bottom of the lower Purbecks, indicating a north and south current. If the 

 depth of water and distance from the shore were sufficiently great for any conclusion to be 

 drawn from these observations, it would be that the stream of the Purbeck estuary trended 

 more from the north towards the west as time rolled on. Perhaps the lithological 

 structure of the beds may be thought to be in accordance with this supposition. 



The association of a minute Serpula with these beds is in accordance with what has been 

 before observed at Durlstone, and shews that the water was probably brackish. On descending 

 lower we come upon a series of softer beds, still of an estuary character. At about ten feet 

 lower down a Corbula and Cardium make their appearance, and six feet lower still are many 

 delicate shells of Leda, Modiola, Ostrea, Cardium, &c. 



We have now about ten feet of clays and sands in layers, the fossils being still the same, 

 with an abundance of Cytherae (for such they must be, associated with Cardium and Serpula,) 

 and then we come to the most valuable bed of stone in the hill, which is a semicrystalline 

 mass of Cardia about four feet thick. . This is a beautiful white stone, and is much used for 

 quoins and other purposes. It will stand well even when set against the bed. 



Three feet beneath this is what I have called the lower insect bed of the lower Purbecks. 

 It contains a great variety of wings, perhaps more than an average number of them being 

 orthopterous. In this bed too is found not unfrequently a small chrysalis, and also a small 

 aquatic insect allied to Notonecta. It appears to accord with the insect bed 116 of Mr Austen's 

 section at Durlstone, where Cardia occur : but there is no actual cardium-rock there. 



* These beds at Teftbnt " lias" limekiln, in the Vale of W ardour, are lithologically very similar, but contain Cyprides and 

 no insects. 



