474 JOURNAL OF THE PLYMOUTH INSTITUTION. 
white clay and pebbles of the Hoe. We have the right to infer 
that wherever the conditions were favourable, the whole range of 
the limestone within the area of the Sound, and the streams 
flowing thereinto, received these deposits.*' 
I hold it to be capable of demonstration that the level wall-like 
character of our limestone — and this is no mere local phenomena, 
but may be observed in Torbay— is due to the action of water ; 
that our limestone ridge is, in short, a platform of denudation, 
formed by a great river which probably followed in the main the 
course of the present Tamar, though it is quite possible that 
ice may have aided in producing this result. The range of lime- 
stone must have been exposed to the denuding action for a 
period of great duration, ere the platform could have been levelled 
as we now find it. While this process was onward, either no 
deposits were thrown down, or they were only harboured in 
sheltered spots. At length there commenced a period of upheaval 
—slow and long continued ; and then the formation of new 
deposits set in. I am inclined to think that the deposition did not 
commence until the crest of the Hoe had been raised nearly to the 
level of the waters. Such fine sand could not have been deposited 
by either a rapid or a deep stream, at any rate at the actual site of 
deposition ; and so with the clay. We see deposits of precisely 
analogous character formed in the present day by the streams which 
flow from china clay works. In the ordinary alluvium, and the 
pebbles intermixed, we have evidently the work of a still later 
period, when the Hoe had begun to peer above the waters, and 
was only liable to occasional overflow, the river meanwhile busily 
eroding the present channels by which it passes the rocky barrier, 
probably in the track of pre-existing fractures and fissures. t 
And now, to return to the bone caverns. Their contents must 
have been carried into them by waters which flowed at a much 
higher level than those of the Tamar and the Plym, or by waters 
which flowed when the land was at a much lower level. Here, 
then, is my hypothesis. The similarity in character and method 
of occurrence of the two classes of deposits, lead me to hold that 
the caverns are in the main contemporaneous with the surface 
* This view of mine has been confirmed, since the lecture was delivered, 
by my attention being called to similar phenomena at Billacombe. One of 
the Yealm Bridge caves was also filled in this way. 
f u Trans. Devon Ass vol. i. 
