XXIII. On the Purbeck Strata of Dorsetshire. By the Rev. Osmond Fisher, 

 F.C.P.S. F.G.S. Fellow and Tutor of Jesus College. 



[Read Nov. 13, 1854.] 



I have been induced to offer the present paper to the Society, because it describes the 

 occurrence of the interesting fossil remains of insects, which it was my good fortune to 

 disinter, and which now form a portion of our noble Woodwardian Collection. 



The occurrence of the remains of insects in the Dorsetshire Purbecks was first observed 

 in 1851, by my friend the Rev. J. H. Austen, of Jesus College, who was then residing at 

 Langton, near Swanage. He was led to search for them in Durlstone bay in consequence 

 of having read Mr Brodie's work on fossil insects. Specimens were subsequently obtained 

 by the members of the Geological Survey, who were engaged at that time upon one of the 

 Dorsetshire sheets of their maps. In the spring of 1852, while on a visit to Mr Austen, I 

 collected my first set of specimens, and I afterwards obtained fossil insects from other 

 localities where the Purbecks appear in the neighbourhood of Weymouth, distant about 

 twenty miles from Durlstone bay*. 



I believe it to be now the general opinion that the Purbeck beds are an estuary deposit 

 belonging to the Oolitic system : and this opinion appears to me to be borne out by their 

 always, as far as I know, accompanying the Portland beds ; for wherever the latter occur 

 they are found capped with the freshwater and estuary limestones and clays of the Purbeck 

 series. This is the case in Wiltshire and Buckinghamshire as well as in Dorsetshire. There 

 is also a resemblance in lithological composition between the Portland beds and the lower 

 portion of the Purbecks, and both have a peculiar bituminous smell when rubbed or struck. 

 Their junction also in Dorsetshire is often so intimate that the same block of stone, as it 

 comes out of the quarry, contains the marine fossils of the Portland series in its mass, and 

 about four inches of freshwater limestone on its upper face full of Cyprides and Cyclas. 



The immediate superposition of the sands and clays of the Hastings series on the Purbeck 

 is a very remarkable fact, shewing that the same area which formed the embouchure of the 

 Purbeck river performed still the same office for the Hastings river. But the entirely different 

 character of the deposit shews at least that the soil of the country drained by the latter was 

 different from that which supplied the former ; and it is evident also that in the district under 

 consideration the motion of the water of the Hastings river was much more rapid, from the 

 abundance of sand, coarse quartz, and gravel, with some pebbles as large as a pigeon's egg in 

 the heavier drifts. 



The specimens were exhibited. 



