350 CAMERON : SUBSIDENCES OVER PERMIAN BOUNDARY. 
connected by short passages or narrow slits in the strata. The 
approach to the uppermost cavity being a subterranean tunnel 
235 yards in length. This tunnel was discovered by the blasting 
of the limestone in an adjacent quarry. The walls of the cavities 
bear signs of having at one time contained water, that having 
rushed along the waterways, whirled about inside, until finally, 
either from the supply ceasing, or being diverted into another 
channel they became empty. Inside were some boulders of igne- 
ous rock, in situ some four miles distant, a few small bones and 
and traces of hematite. The surface of the hill is bare limestone, 
very much fissured and channelled, abounding in the hart's tongue 
fern. 
In Furness the hematite is found in " sops " or pockets in 
the limestone, which if they happen to be at the surface, admit of 
the ore being dug out with a spade as clay is at a brick yard. 
The "sops" maintain a determinable course though they are often 
far between each other, and are probably swallow holes that 
formed when the limestone was as yet uncovered by the ferrugi- 
nous sandstone that afterwards filled them up with ore, or they 
may in some way be the result of the bulging out and contrac- 
tion of the hematite itself. 
Subsidences have recently occurred at Blackheath, near 
London. They first began in 1878, after an extraordinary flood, 
more appeared in November last. They are from fifteen to 
twenty feet deep, and are in the Pebble Beds overlying Chalk, 
that constitute the Blackheath plateau, near to the river. 
Mr. T. V. Holmes, F.G.S., has given interesting accounts of these 
pits in the" Engineer," March, 1881 ; and Mr. De Eance, F.G.S., 
in " Nature," in February, 1881. From these we learn that the 
water level of the chalk is considerably below the bottom of the 
pits, and the gravel too thick to admit of them being pipes in 
the chalk. 
Mr. T. V. Holmes, late of the Geological Survey, has for- 
warded me the Report of the Committee for the Exploration of 
