Mr FISHER, ON THE PURBECK STRATA OP DORSETSHIRE. 563 



Ft. In. 



Cream-coloured marly limestone in three beds ... I 3 Insects (e) 



— ... 8 Insects (d) 



... 1 Insects (c) 



Softer and more clayey 4 Insects 



Blue shaly marl 1 



Hard cream-coloured marlstone 1 2 



Rubbly marls 3 



Laminated sandy shale with chert 8 Cerithea 



Rubbly marl 3 



Grey marly limestone in two beds 2 Insects (b) 



This section shews a succession of conditions of the ancient surface, several times returning 

 to a state favourable to preserving these structures. In some of the bands they are much more 

 sharply preserved than in others, — the most delicate markings being distinct, and the smallest 

 wings perfect. In others they have a sodden appearance, as if they had been washed about on 

 the surface of the ground before a fresh layer of mud entombed them. The clubs of the antennas 

 of large beetles occur, with segments and other portions of the bodies, but the most abun- 

 dant remains are elytra and wings. All the orders are represented*. The insects are mixed 

 with fragments of carbonized matterj- ; and it is very unusual to find the one without the 

 other. They lie in laminae throughout the blocks of marl, sometimes one, sometimes several 

 in a block ; but wings, especially the larger ones, are met with dispersed throughout the 

 stone. It is evident that the wings and other fragments have been washed together into 

 patches ; and, coming upon such a patch, the collector has a rich treat, until it is exhausted, 

 when he may search long comparatively in vain. In short, the fewer veins in a given 

 thickness of the marl the richer they generally are — representing a longer period of depo- 

 sition (but still, as I conceive, a very brief one for every vein). If the particles of mud 

 were kept in a state of commotion the insect and vegetable remains would continue at the 

 top until a quiescent moment allowed of their being buried. As might be expected, the 

 elytra almost invariably lie on their convex surfaces, and the blocks which contain the 

 specimens are from the upper side of the vein. The elytra at Durlstone do not shew any 

 parallelism of arrangement, from which it may be inferred that the current was imperceptible. 

 It is otherwise at Ridgway. 



The deposit in the bed marked (f) is rather peculiar. The carbonized fragments are 

 coarser, and the elytra retain their coloured markings in a singular degree. This is probably 

 due to the stone being more aluminous than usual. 



Since it is probable that the insects were blown or washed into the estuary only during 

 the latter part of the summer and the autumn, the veins in which they occur assume a curious 

 chronological value, and an interesting question of the rate of deposition of the beds arises, 



* I found a minute but very perfect lepidopterous insect in the upper Has at Ilminster, Somerset; it is now in the Wood- 

 wardian collection. 



+ Professor Henslow thinks that these are for the most part portions of insects in different stages of decomposition. 



