The Geological Survey of England and Wales. 391 
of from 40 to 50 per cent, of the stone. The upper division, 
about 70 feet thick near Devizes, consists of green and grey sands. 
As these are irregular in thickness, thin out rapidly to the north, 
and extend as a band in a nearly east and west direction, they may 
represent an ancient sand-bank. The persistence of the Malmstone 
over a very wide extent of the Upper Greensand of England is a 
noteworthy fact. 
A revived industry of some interest on the borders of Bedford- 
shire and Buckinghamshire is the extraction of fuller’s earth from 
the Lower Greensand. This deposit is now worked by mines on the 
flanks of the escarpment. Mr. Cameron has frequently visited these 
mines, and has described them in papers read before the British 
Association and elsewhere. 
Jurassic . — Some of the most important recent additions to our 
knowledge of the structure of the Jurassic and Cretaceous rocks of 
the South of England have been made by Mr. Strahan in his re- 
examination of Dorsetshire for the Drift Survey. The area known 
as the Isle of Purbeck has long had a peculiar geological interest, 
not only from the fact that the Portland and Purbeck rocks there 
reach their maximum development, but also from its structure. It 
is traversed by an extremely sharp and faulted monoclinal fold, a 
continuation of the Isle of Wight monocline, from which, however, 
it differs in being accompanied by inversion of the strata and much 
overthrust faulting. This structure may, in fact, be regarded as an 
intermediate stage between a simple monocline and a complete over- 
thrust. The deeply indented coast affords unusual facilities for 
examining the effect of the movement. The old one-inch map, on 
account of the smallness of the scale, gave merely a diagrammatic 
view of the structure of the “island.” In the re-survey on the six- 
inch scale both the faults and the subdivisions of the strata have 
been traced with a detail that was before impossible. In the Isle 
of Purbeck the principal additions to the map consist in the tracing 
of the subdivisions of the Cretaceous system. The Lower Green- 
sand, which is so well developed in the Isle of Wight, was known to 
exist in the Isle of Purbeck also, but its limits had never been 
determined. It has now been separated from the Wealden group, 
with which it was formerly confused, and it has been traced west- 
ward until it finally thins away, while at the same time the Wealden 
Shales, which form the uppermost subdivision of the Wealden group 
in the Isle of Wight, have been traced through the Isle of Purbeck 
as far westward as they extend. 
During the mapping of the Lower Greensand, some interesting 
evidence as to its relation with the overlying Gault came to light. 
This evidence tends to confirm the conclusions formed during the 
re-mapping of the Isle of Wight, for the break at the base of the 
Gault, which was there only suspected, becomes so much more pro- 
nounced westwards as to suggest that the base of the Cretaceous 
system might have been more suitably drawn at the bottom of the 
Gault than at the bottom of the Wealden group, which is in- 
separably connected with the Purbeck Beds, Moreover, a con- 
