346 CAMERON : SUBSIDENCES OVER PERMIAN BOUNDARY. 
walls of this ravine the subsidences take place more frequently 
than elsewhere, this may be due to the strata being in gTeater 
dilapidation from the above causes, areas less compact than others 
determining the locality of the pits. 
What appears to me »a more probable explanation of the 
origin of these curious subsidences is, that where the under- 
ground water flowing over the limestone surface, reaches the 
margin of the sandstone, it receives a check whereby it accumu- 
lates, forming a chain of dams or pools along the line of junction 
of these rocks. As denudation proceeds, hollows will form above 
and below, until ultimately the phenomenon of the pits appears. 
Should this be so, 14 the water bubbling up and frothing all over" in 
newly formed pits, and in wet seasons in those that at other times 
are dry, is easily explained, without calling in the aid of the 
river, as after great rains, the superabundant water that has 
accumulated below would rise to the surface by these vents. 
It may be worthy of note here, that in wet seasons at \Vells. 
near Bedale, water runs off a limestone hill, on to a gravel flat, 
where it disappears almost at once ; after a while, a number of 
dry basin shaped holes in the gravel become filled with water. 
A large swallow hole in Ripon is called Salt Pit, but I have 
no history of it, 
The " Hell Kettles " at Croft are two ponds connected by a 
narrow channel on the surface, situated near to and on an allu- 
vium of the Tees, the water in them may be Tees water, that after 
traversing the limestone, is conveyed to the pit mouth of these 
natural shafts or swallow holes. The Hall Garth Ponds, on a 
terrace of the Ure are very similar cavities the water in them 
rising and falling with the river. 
That something very like a Ripon swallow hole did make its . 
appearance where the " Hell Kettles " are, may be inferred from 
the following quaint description of a " convulsion of the earth," 
by a twelfth century annalist. 
