CAMERON : SUBSIDENCES OVER PERMIAN BOUNDARY. 343 
the wasting away of the limestone itself. The probability is, 
that the last named theory is the most correct, for while gypsum 
is by no means readily dissolved, limestone is readily soluble 
in water charged with carbonic acid. Where sulphate of lime is 
dissolving to any extent amongst the rocks, one might naturally 
expect sulphuretted springs in abundance, whereas there are none, 
except one now disused and closed up, called the ' Spa well,' 
near where the Ripon gas house is built. On the west bank of the 
TTre, opposite Plaster Pit House, there are fine sections of very 
hard compact masses of gypsum, their crushed contorted appear- 
ance being more likely due to the effects of faults passing near 
than to the river action. Rain then falling on the fissured and 
porous surface of the strata, quickly percolates beneath, and by 
its dissolving action, widens the vertically inclined joints, honey- 
combing it, and leaving in time nothing, it may be inferred, 
but slender pillars, producing a net work of cells. " A moment 
arrives when the cohesive force of these pillars is less than the 
weight of superincumbent rock," (see Prof. Newcomb on the 
Course of Nature Xo. lxi Jan. 1879) the pillars fall, followed by 
the roof they support, the collapse causes at the surface open 
shafts or pits, engulphing whatever may be on it at the time. 
This is in every way specially narrative of the falling in of 
the Ripon pits, for Mr. Tute tells us that while some farm labour- 
ers were at dinner, a hole appeared and swallowed up the hay 
stack they had been building. 
Most of these dangerous holes have appeared suddenly, and 
without any noticeable disturbance of the surface beforehand. 
There are cases in which shallow depressions first form gradu- 
ally on the surface, these have been noticed to sink 4 or 5 inches 
in a year. Narrow well-like holes, locally known as " man holes," 
also appear at intervals, two or three together in one field. These 
come daring wet seasons, and at first contain water, it is thought 
that a field in turnips after being cleared by sheep is more liable 
to them than others. 
