104 Geology of Wiltshire. 
which, from the large development their continuation obtains 
further south, in the isle of Portland, are called Portland oolite 
or sand. These beds are extensively quarried at Fonthill, Tis- 
bury, and Chilmark in the Vale of Wardour, whence was ex- 
tracted the stone of which Salisbury Cathedral is built. Here 
there appears a considerable fault or fracture in the strata along 
the southern foot of the chalk escarpment, and the beds of Portland 
stone have been tilted up by subterranean disturbance. Some of 
them dip at an angle of about 40 degrees towards the north and 
east. The Kimmeridge clay is a considerable formation, having 
a thickness of from 300 to 400 feet. The Ostrea deltoidea is one of its 
most characteristic fossils. In the Portland stone and sand are 
found numerous specimens of the Trigonia gibbosa, (fig. 16), and 
occasionally corals, such as Isastreea oblonga, (fig. 17). 
Fig. 17 (345). 
Fig. 16 (346). 
AAU nt 2 : 
Trigomo gibbora, . 4 Dat. ize. ' Isastreea oblonga, M. Edw. and J. Haime. 
As seen on a polished slab of chert from 
Portland Sand, at Tisbury. 
The organic remains contained in some of the strata immediately 
above the Portland stone, are referable to species that must have 
inhabited fresh, or at all events brackish water. They were, there- 
a. the hinge. 
Portland Stone, Tisbury. 
fore, probabiy, deposited in an estuary, forming the mouth of some 
river. In the isle of Purbeck in Dorsetshire, this freshwater cha- 
racter is more strongly marked than in any of our Wiltshire beds, 
and in the Weald of Sussex and Kent a great series of beds, con- 
sisting of limestones, sandstones and clay, are met with belonging 
to this geological period; but this ‘Wealden’ formation, as it is 
called, is wanting in Wiltshire. The Portland beds and Kimme- 
ridge clay form the highest of the Oolitic series, the Upper oolite 
of the Geological Survey. 
