TRANSACTIONS OF THE SECTIONS. 93 
On the Phenomena exhibited by the Plastic Clay Formation in the 
vicinity of Poole, Dorsetshire. By the Rev. W. D. CLarKe. 
Mr. Clarke stated that it was his object to show that the plastic 
clay had partaken of the movements and dislocations which had rup- 
tured the chalk and subjacent strata; and further, that the denuda- 
tions which the plastic clay had experienced were also dependent on the 
process by which the bearings and fracture of the beds had been 
affected ; or, in other words, that aqueous denudations had here been 
the result of subterraneous movements, and not of any independent rush 
of water, such as a diluvial current. The plastic clay series, near 
Poole, consists of beds of sand and clay, the pipe clay containing much 
alum, iron, and manganese ; and having, at an ancient date, been exten- 
sively worked. Many of the sands rival in intensity of colour those of 
Alum Bay, and some, as at the red rock of Studland, are highly inclined. 
Between Purbeck and the Wiltshire Avon the whole country consists of 
hills of sand and clay, interspersed with valleys and deep furrows, there 
being apparently not a particle of calcareous matter. The district is 
completely barren, and curiously wild ; vegetable remains are extremely 
abundant, chiefly in the marls or clays, sometimes in a hard siliceous 
sand, which has been cemented into a sandstone by ferruginous matter. 
Some of the beds of sand have been so firmly cemented as te afford 
very good building stones, and Mr. Clarke thinks that the greywea- 
thers of Wiltshire, of which Stonehenge is composed, were portions 
of beds belonging to the plastic clay. Mr. Clarke remarked that 
specimens of sand from a depth of 148 feet (or 138 feet below the 
harbour level) contained flint, quartz, slate, and wood; and that it 
was consequently a drift sand, brought, as he believed, by currents 
from the West. The probable thickness of the plastic clay is 600 
feet, and there can be little doubt that it originally filled up the basin 
in which it now lies, having occupied the bed of some lake or gulph, 
—the sea, the boundary of the ancient beach, being apparently mar- 
gined by a bed of pebbles. Since its deposition the plastic clay has 
been scooped out by the action of water, so that the whole country is 
_eut up into a great number of hills and hollows. The harbour of 
Poole occupies one of these valleys, the sea having occupied a great 
depression in the line of the Purbeck range, which appears to have 
been the result of two faults, the space between them having dipped 
outwards. Many deep, dry furrows are met with along the edge of this 
descent. The furrows on the surface are neither ascribed by Mr. Clarke 
to diluvial action nor to the action of springs; but are considered as. 
primarily cracks, or fissures, the consequence of elevatory disturbances, 
or of faults. From a comparison of the phenomena exhibited by the 
eracks, faults, and joints of the chalk and plastic clay beds, Mr. Clarke 
comes to the conclusion, that the plastic clay was elevated with the 
chalk, either at once, or by successive impulses of the subterranean 
forces ; and consequently that the strata below the chalk have in the 
south of England been elevated since the tertiary period. 
The gravel which caps the hills and islands consists principally of 
