Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 559 



has given the name of the " broken shell limestone." It is composed of shells of a species 

 of Cyrena, and attains a thickness in Durlstone bay of no less than nine feet. Next suc- 

 ceeds a series of about twenty-five feet of shales and clays, with layers of crushed bivalve 

 shells and cyprides. Among these beds occur bands of fibrous carbonate of lime, called 

 locally "beef" and "horseflesh." The crystals of this mineral are usually found shooting 

 upwards from a band of perished bivalves, and appear due to a change in the condition 

 of the shells on which they rest. The connexion of these bands of "beef" with deposits 

 of bivalve shells is most strikingly seen in the neighbourhood of Osmington. 



Next we come upon a mass of strata containing bands of apparently marine origin, 

 interstratified amidst others still denoting the prevalence of a brackish condition of the 

 estuary waters. These marine bands abound in a species of Corbula, with at least three 

 species of Melania (or perhaps rather Ceritheum), a Protocardium, &c. These " Corbula 

 beds" attain the thickness of about eight feet. 



We now descend upon a portion of the deposit decidedly marine, as the change in the 

 very appearance of the rocks at once leads the observer to suspect. The stone becomes 

 earthy, coarser, and more rubbly in its texture, and the shells are Pectens, Oysters, Cardium, 

 Perna, Modiola*, &c. ; and it is amongst these that we find a marly limestone, sometimes blue 

 and sometimes olive brown, the first band containing insect remains, — a remarkable fact, as, 

 a priori, I do not think we should have sought for these fragile and terrestrial forms, where 

 both marine and fluviatile beds exist, by preference amongst the former. Yet it is singular 

 that hitherto all the chief insect limestones of the Purbecks have been found in the marine 

 parts of the series. 



The rocks of this part of the series are many of them of a dark greenish hue ; others 

 brown ; and towards the bottom of them Corbula bands again occur, as we approach thefresh- 

 water beds beneath. One of these often exhibits slabs, covered with a fine Paludina, so 

 striking in appearance as to arrest the attention of the quarrymen. The total thickness of 

 these "Pecten beds" is about forty feet. 



A very important division of the middle Purbecks is the next in descending order. It is 

 that from which all the best stone is quarried, and in which the abundant and interesting fish 

 and turtle remains are chiefly obtained. The Macrorynchus also occurs in this horizonj-. The 

 limestones and shales of this portion are chiefly of freshwater origin, and many of the beds 

 are formed of the shells of Cyclas. Traces, however, of an influx of estuary waters are seen 

 in the lowest bed but one, in the occurrence of Ceritheae, Ostrese, &c. ; but the subdivision ends 

 with a purely freshwater deposit called the Downs vein, and composed of Cyclades. This is 

 the vein from which most of the fish are obtained. These beds, in all about forty-four feet, 

 rest upon a band of shale, covered with the Cypris fasciculata, and full of Cyclas on the 

 under side. 



The Archaeoniscus, which for many months eluded all observation at Durlstone bay, was 



* I am informed by Mr Bristow that these marine shells of t A vei 7 fine collection of these fossils is in the Museum at 



the middle Purbecks are of decidedly Oolitic types. i Dorchester. It formerly belonged to Mr Wilcox of Swanage, 



| and is referred to by Professor Forbes in his paper. 



Vol. IX. Part IV. 72 



