THE BI.ACKHEATH SUBSIDENCES. 
241 
an old shaft leading to a cavern formed (probably in the chalk) 
by the band of man. A third view appears to be entertained, 
which I mention for that reason only, though it has neither evi- 
dence, nor even possibility, in its favour. It is that ‘ the Black- 
heath subsidences are due to the action of water, the sand 
being carried off and leaving the pebbles to collapse.’ {Time. s\ 
May 12, 1881.) The holders of this view appear to forget that 
the formation of natural underground cavities in highly siliceous 
sand and gravel is impossible, inasmuch as they, unlike the 
Chalk, remain utterly insoluble when exposed to the action of 
rain-water charged with carbonic acid gas. I need therefore 
say no more about this third view, hut will consider the opinion 
first mentioned, the only geological view which deserves the 
name. 
Allowing 35 feet as the depth at which the top of the shell - 
beds would be touched, in unbroken ground near the hole, the 
beds thence to the top of the Thanet Sand must be quite 18 feet 
thick. The Thanet Sand carniot be less than 40 feet thick, and 
may be 5 or 10 feet more. Thus, at the lowest computation, 
the distance to the Chalk would be 93 feet. Mr. W. Whitaker, 
however, who has probably had a more extensive experience of 
these beds than any other living geologist, remarks that while 
a few feet only of Tertiary beds above the Chalk give rise to an 
uneven surface, and to pipes, the same process of the dissolving 
of the Chalk by carbonated water ‘ when it goes on at greater 
depths seems to act in a more equal (because in a more constant) 
way on the surface of the Chalk, leaving it even.’* Now Mr. 
Whitaker’s experience must have been gained in pits and rail- 
way cuttings that would seldom show even 50 or 60 feet above 
the Chalk, much less 80 or 90 feet. Again, Mr. De Ranee 
observes f that the permanent water-level in the Chalk at Black- 
heath is about Ordnance datum, and that the comparatively 
small amount of chalk there above this water-level makes it 
very unlikely that these subsidences are due to pipes descending 
vertically into the Chalk. Lastly, our own experience of the 
way in which water falling on the surface of Blackheath is held 
up by the shell-beds below, and thrown out just above their 
line of outcrop, is conclusive evidence that scarcely any water 
can ever get down, to the Chalk below at all. All that does do 
so (artificial holes apart), must fall as rain outside the area of 
the Blackheath plateau. 
An interesting letter appeared in the Engineer of Feb. 18, 
1881, signed Alfred W. Morant, in which the writer gave 
an account of some remarkable subsidences which appeared in 
great numbers many years ago on the sewage farm at Whit- 
lingham, near Norwich. He says : ‘ When the sewage was first 
* Geology of London, 3rd edit. p. 32. t Nature. Feb. 17, 1881. 
NEW SERIES, VOL. V. NO. XIX. K 
