By A. J. Jukes- Browne, B.A., F.G.S. 



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grey and green sands, containing layers of hard greenish rock ; 

 these are very conspicuous in the road cutting south of Devizes, hut 

 we need only mention one peculiar fact about them, and that is that 

 though they are about 70ft. thick near Devizes, they thin out so 

 rapidly northwards that there are only 3ft. or 4ft. of such sand at 

 Heddington and Calstone. They form, in fact, a huge sand bank, 

 which seems to run nearly due east and west along the Vale of 

 Pewsey. 



The silicified sponges which are found at several places in the Vale 

 come from these sands, and they are also the home of several species 

 of Pecten, especially P. asper, P. orbicularis, and P. intersiriatus, 

 but other fossils are not abundant. In the Vales of Warminster 

 and Wardour these green sands contain layers of Chert and Sponge- 

 rock. 



5. Lower Chalk . The passage from Green sand to Chalk is rather 

 rapid, the passage beds being only 6ft. or 7ft. thick, and consisting 

 of sand embedded in chalky matter, the proportion of the latter 

 gradually increasing till the grains of sand are few and far between. 

 In these beds are a number of greenish or brownish nodules, many 

 of which are casts of fossils, so that the rock looks very like a 

 greenish sandy mortar stuck full of pebbles. It is well exposed at 

 Calstone and at Urchfont, and is generally known as the Chloritic 

 Marl. 



In the overlying Chalk the green grains gradually die out, and 

 we find a light grey kind of Chalk, some parts of which are soft and 

 some are hard. In other parts of the country this lower Chalk is 

 often burnt into lime, but in this part of Wiltshire it is never burnt 

 because it does not make good lime ; clearly, therefore, there is 

 something peculiar about it, and this peculiarity is soon found when 

 it is examined under the microscope. When highly magnified 

 it is seen that the material is not all carbonate of lime, but that 

 part of it consists of minute discs and globules, exactly like those in 

 the Malmstone previously described, only they are smaller ; and, as 

 before, whenever this globular silica occurs the siliceous spicules of 

 sponges are also abundant. This curious siliceous Chalk has been 

 found at Compton Bassett, Stockley near Calstone, Heddington, 



