562 Mr FISHER, ON THE PURBECK STRATA OF DORSETSHIRE. 



Durlstone Bay. Ridgway. 



Ft. Ft. 



Broken shell limestone 9 10 



Beef 24 3 



Corbula beds 8 IS 



Pecten beds 34 6 



Turtle beds 44 7 



Cinder 12 7 



Cherty and marly beds 30 11 



161 57 



We now return to Durlstone bay, and describe the Lower Division of the Purbecks. 

 The upper portion of these beds has been deposited in salt water, but evidently near 

 the coast. The lower part of them is fresh water. They commence in descending order, 

 with a series of marly and shaly bands, after about eleven feet of which we come upon a blue 

 marl, containing a small species of Serpula in great profusion. The insect beds succeed to 

 this, and demand a more particular description. 



The insect beds consist of a series of rather soft marly limestone bands, alternating with 

 shaly clays. They are evidently sedimentary, and scarcely contain any vestiges of shells. 

 I have indeed never seen more than one (Hydrobia ?) in them myself. This is remarkable 

 among the Purbecks, where so many of the beds are entirely made up of shells. The 

 stone abounds in cavities of a cubical form, with the sides somewhat bulged inwards*, 

 formed apparently by crystals of common salt, which have been dissolved out, and 

 their places sometimes partially filled by carbonate of lime, with a trace of sulphate of 

 barytesj-. They lie in horizontal layers, and. seem to point to the stone having formed the 

 muddy surface of a saltern, subject at times to be dried slowly, so as to admit of the 

 crystallization of the salt with which it was imbued. I dissolved a portion of the bed from 

 Bidgway in nitric acid, and found the glass in which it stood so much corroded, that fluoride 

 of calcium must have been present. The following is a section of the upper insect beds of 

 the lower Purbecks at the most productive point in Durlstone bay. 



Ft. In. 



Grey marls with clay interstratified. 



Blue marl with serpula 1 6 



A parting of yellow clay Oh 



Grey marly limestone in three beds 9 



6 



9 Insects (/) \ 



Thinner bands of marl 2 



* See Ormerod's paper on pseudomorphs of salt in the new red sandstone of Cheshire.— Geol. Journ 1853, p. 187. 



+ I am indebted to the kindness of Professor Miller for this account of the crystalline cavities. 

 J The letters refer to the labels on the specimens. 



