CAMERON : SUBSIDENCES OVER PERMIAN BOUNDARY. 349 
of salt only a very short way from Hartlepool, and it is possible 
that when the coast line extended further into the present bay, 
the drainage off the land may have found access to beds of salt 
below and dissolving them, caused a cavity into which the over- 
lying strata has sunk, giving rise to the large pool or morass 
called the Slake. A hollow of this kind would retain much mois- 
ture on its surface, so that water plants would grow in great 
profusion, and in course of time would cause the hollow to partly 
fill up and become a marsh. 
Of other subsidences than those connected with the Permian 
boundary, Mr. Fox Strangways, F.G.S. tells me that in the 
Middle Calcareous Grit, on Dalb}^ Moor, near Pickering, there are 
circular holes about ten feet deep, which are caused, he believes, 
by the solution of the calcareous beds below, most of the 
springs in the valley beneath being highly charged with calcareous 
matter and forming great deposits of tufa. These sink holes, if 
they get grown over with bracken are a source of some anxiety 
to huntsmen in the district. I am told that an old huntsman in 
the service of a gentleman at Thornton, rode into one of them, 
when returning home in the dusk from the hunt. Luckily for 
himself and horse the pit was a shallow one. 
There are a collection of shallow pits on Brompton Banks, 
Northallerton, called " Soldier Pits," considered by the people to 
have been places made for defence, but there is more reason to 
believe they have been workings for gypsum or other mineral. 
The Rev. Mr. Atkinson, of Danby, considers the pits about 
the base of Roseberry to have been the results of former mining 
enterprise, and that many of the alleged British villages may have 
a like explanation. 
In Geol. Mag., Vol. viii, page 312, July, 1871, I have 
described a large swallow hole or cavern in the carboniferous 
limestone at Stainton-in-Furness. Hidden away in a limestone 
hill are three large dome shaped cavities, one over the other, and 
