THE BLACIvHEATH SUBSIDENCES. 
247 
of two or three spliced together, as I am informed is the usual 
mode in Bolivia. 
It seems likely that the upper third of the narrow shaft 
became filled up mainly by the falling in of the pebble beds, 
leaving the lower two- thirds almost or quite empty. (Possibly 
a very slight hollow without any outlet, about 30 feet in 
diameter, and perhaps 1 ft. 6 in. deep at the centre, which 
exists on the west side of the hole at the surface, marks the 
amount of the material that thus filled the upper part of the 
shaft.) And a little reflection shows that the line of junction 
of the pebble beds with the clay beds below, is the spot at 
which the incessant trickling of water down the neglected 
shaft could not fail to produce, sooner or later, subsidences 
precisely resembling those which have appeared ; for the effect 
of the water concentrated towards the base of the pebble beds, 
would be to undermine them and wash the disintegrated sand 
and pebbles down the shaft, together with a considerable but 
smaller amount of the subjacent clay. In this way a hollow 
would gradually be formed, conical in form, from the base of 
the pebble beds upwards, while the mouth of the shaft below 
would acquire a broad funnel shape, from the denudation of 
the upper surface of the clay. The Thanet-sand and chalk 
part of the shaft would probably be little altered, and the chalk 
cavern below would allow of the percolation through it of any 
quantity of water, and be at the same time more than spacious 
enough to receive any quantities of sand, gravel, or clay, the 
removal of which would suffice to cause a subsidence at the 
surface such as we have endeavoured to examine. 
Note . — As the Subsidence Committee are by no means 
unanimous with regard to the origin of these holes, it has been 
resolved that in their forthcoming Report the various opinions 
held shall all be represented. — T. Y. H. 
