98 Geology of Wiitshire. 
and Highworth in the north east, to Bradford and Westbury at the 
south western extremity. The softer beds of the series, called the 
Oxford and Kimmeridge clays, being more easily worn down and car- 
ried away by rains, naturally occupy the lower levels of the vale ; 
while the harder beds (Coral-rag, Forest marble, and Oolitic lime- 
stone) form successive terrace-like ridges or ranges of hill, stretch- 
ing nearly north-east by south west across the north-eastern 
corner of the county. 
I will now briefly trace the situation and distinguishing charac- 
ters of these different strata, beginning with the oldest, or under- 
lying beds on the western boundary of the county. 
It has been said that the oldest strata in the county are the upper 
lias shale and marlstone, which are by many geologists considered 
to belong to the same group as the oolitic, or (as it is termed by 
continental geologists, from its composing the mass of the Jura 
mountains) the Jurassic series. But these lias marlstone beds only 
occur in the bottom of the deep valley of Box, where it joins that 
of the Avon, barely within the boundary of the county. They are 
immediately covered by the sands and limestone beds of the inferior 
oolite, which form the substratum of the entire range of the Cots- 
wold hills, bordering the Vale of Severn from Bath northwards 
through Gloucestershire into Worcestershire. In Wiltshire the 
inferior oolite only shews itself in the escarpments of the valleys of 
Box and Bradford, being immediately covered by beds of marl and 
clay, called ‘Fuller’s earth,’ and these by the ‘ great oolite’ or Bath 
freestone, which caps the hills about Bradford, Box, Colerne, and 
Castle Combe, and is in turn very soon overlapped by the more fissile 
limestones, which are called Forest marble and cornbrash. The uni- 
ted thickness of these beds composing the lower oolitic group of the 
the Geological Survey, varies, perhaps, from 300 to 500 feet. Some 
beds of the middle part of the great oolite are several feet in thick- 
ness without a joint, and from them are quarried those magnificent 
blocks of cream-coloured freestone, which are now transported by 
railway from the neighbourhood of Box to London and many other 
parts of England. In the mere sketch, for which alone we have 
room in this place, it is impossible to give full lists of the fossil 
