By 4. J. Jukes-Browne, B.A., F.G.S. 323 
grey and green sands, containing layers of hard greenish rock ;: 
these are very conspicuous in the road cutting south of Devizes, but 
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. interstriatus, 
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 Greensand 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 Wiltsbire 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, 
