J 50 Proceedings of Philosophical Societies. [Feb, 



north, appear to rest on a bed of calcareous blue clay, provincially 

 called gait. This bed, covered in some places with gravel, forms 

 the extensive flat which connects the upper parts of the country 

 with the fens. 



The thickness of the gait bed, judging from the depth of the 

 wells which have been sunk in it, is probably not less than 200 feet, 

 ^nd it is considered by the Professor as the lowest bed of the chalk 

 formation. Above this bed lie several strata of a chalky stone, 

 called chinch, which is an argillaceous lime-stone in great request, 

 ^nd of which large quantities are dug and burnt at Cherry Hinton, 

 and other places irj the county. The clunch contains no flint, but 

 abounds in the common radiated pyrites. On some of the highest 

 of the hills near Cambridge is found a deposite of gravel and loose 

 stones in horizontal layers, which difi^ers in so many respects from 

 the gravel of the subjacent flat country that it must probably be 

 considered as the deposite of another epoch. It consists principally 

 of pale blue flint, enclosing numerous traces of ascidia, but also 

 contains fragments belonging to the oolite formation of the neigh- 

 bouring counties of Northampton, Rutland, and Lincolnshire, with 

 occasional fracments of basaltic rocks. 



The characteristic fossil of the clay or gait bed is that variety of 

 belemnite described by Lister under the name of lapis lyncurius» 

 There have also been found an obscure organic remain, considered 

 by the Professor as a medusa or beroe ; also a beautiful ammonite, 

 ^ pentacrinite, and several species of bivalve shells. The mutilated 

 remains of a species qf fish have also been found j and charred wood 

 ^s not uncommon among the anirnal remains. 



Among the rarer fossils of the Cambridgeshire chalk, are fruit 

 pones and linear leaves from Cherry Hinton. 



The results of an analysis by Mr. Holme of three varieties of 

 Cheny Hinjton lime-stone are subjoined, from which it appears that 

 the stone consists for the most part of carbonate of lime, but also 

 contains from aboijt fqur to nine per cent, of alumina, and a portion 

 of silex not aniounting to two per cent. 



Tyvo letters addressed to the Secretary from the Rev. Richard 

 Hannah, jun. Chaplain tq the troops at Plymouth, were read. 



Spme geologists having doubted or denied the existence of 

 prganic remains in the lime-stone of Plymouth, the object of Mr. 

 Hannah's letters is to state that both madrepores and shells have 

 recently been found by him in the quarries of Stonchouse Hill, and 

 at the Dock-yard ; a fact entirely corroborated by the specimens 

 which Mr. H. has also transmitted for the cabinet of the Society. 



Some observations on the interruptions or idu\is to which mineral 

 vpins are liable, contained in a letter addressed by Mr. L Sadler to 

 Ant. Carlisle, Esq. M.G.S. were read. 



The cases solved in this paper are illustrated by diagrams, which 

 renders it impossible to give an abridgment of them. 



Dec. 2. — A letter from L. Horner, Esq. to the Secretary was 



