564 Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 



and this is none the less interesting from the fact of different sets of wings being found in the 

 different veins. The scattered wings which are found throughout the blocks were probably 

 casually blown or washed into the stream, but the abundant deposits in the veins seem to 

 indicate periods of great destruction of insect life, such as would be occasioned by heavy 

 storms or cold weather setting in. If, as might be expected, each autumn were represented by 

 its consignment of these beings to their burial, the number of years indicated by the upper 

 insect bed of the lower Purbecks would be very small — about four. But if the different 

 tribes found in the different beds were those which perished at different seasons of the year, 

 and each season were represented, then the time becomes incredibly short — not more than two 

 years for the particular beds now under discussion, one for each set of beds. There is how- 

 ever nothing difficult of conception in a purely sedimentary series of beds like these being 

 rapidly deposited, as there would be in the case of strata made up of the exuviae of animal 

 life. The portion of the insect beds which is the richest is at a point where they are thicker 

 than usual, and lie somewhat hollow. I believe this to be their original form, and not to be, 

 at any rate not wholly, due to a subsequent flexure. Such a form of the estuary bottom would 

 be favourable to the collection of the remains into patches, and would account for the same 

 locality being rich in several veins in the same vertical. The quarrymen maintain that they 

 usually find fish in similar hollows in the strata. 



Below these upper insect beds of the lower Purbecks there follow about fifty-five feet of 

 shaly clays and marls containing estuary shells, and in this part of the series there occur 

 blocks of gypsum, which are quarried for plaster. The strata are much disturbed where the 

 gypsum occurs: but the disturbance appears connected with the chemical change which 

 produced the gypsum, the limestone beds above the gypsum retaining their regular arrange- 

 ment. We then meet with another bed of cream-coloured marly limestone (a), containing 

 insects, associated with layers of small Cardia and Hydrobiae. It was from this bed (116 of 

 Mr Austen's section) that the very beautifully preserved hemipterous insect in the Wood- 

 wardian Museum was obtained. This is the lower insect bed of the lower Purbecks, but 

 elytra and small wings associated with vegetable matter continue to occur occasionally 

 whenever the strata are of a texture favourable to their preservation. The Archaeoniscus 

 has been found in this last-mentioned stratum. 



About sixty feet of marls, Cypris-shales, and, at the bottom, more calcareous and sandy 

 shales, complete the series as displayed at Durlstone bay. There is a bed of olive-brown and 

 blue indurated marl about the middle of these, in which the Archaeoniscus is tolerably 

 abundant. It also occurs in the Cypris-shales. The junction of the Purbecks with the 

 Portland beds is not well shewn in this locality. In the middle of the bay a fault cuts off" 

 the series before the bottom of the Purbecks is quite reached. The beds are then repeated 

 from the middle Purbecks to the Portland, but at the southern end of the bay, where the 

 Portland oolite appears, there is so much disturbance that it is impossible to carry the section 

 through satisfactorily. The disturbance is a general one affecting the Portland as well as the 

 Purbeck beds, and is not solely an example of the peculiar condition in which the lower 

 beds of the Purbecks appear at other places in our district. But that broken state of the 

 strata is very well seen in the Cliff beneath Round down, about a mile to the west. The 



