669 
The following Papers were then read : — 
ON THE STRUCTURE AND ORIGIN OF THE MILLSTONE-GRIT 
IN SOUTH YORKSHIRE. BY H. C. SORBY, ESQ., F.R.S., 
F.G.S., &C. 
The nature of the materials of which stratified rocks are 
composed is not only interesting on its own account, but 
especially so as enabling us to form some opinion on the 
geological characters of the ancient land from the waste 
of which the materials were derived. It is for this reason 
in particular that I now purpose giving a short description 
of some facts I have ascertained with respect to the mill- 
stone-grit in South Yorkshire. In many places this rock 
is a very coarse-grained sandstone, being often made up of 
grains ranging up to J^th of an inch in diameter, along 
with which are rounded pebbles, whose size varies up 
to fully one inch in diameter. 
In order to ascertain the laws governing the drifting 
forward of sand and pebbles along the bottom by a current 
of water, and those involved in their final deposition, I 
made a number of experiments some years ago, which I 
briefly described in a paper read before the British Asso- 
ciation at Glasgow, in 1855. I there showed that, if there 
be a surface of moderately coarse sand, like that composing 
the millstone-grit, rounded pebbles do not sink into it ; and 
that the resistance they experience in moving over its surface 
is no greater in proportion to the motive power of the 
current than in the case of grains of sand ; and, therefore, 
a current moving with sufficient velocity to drift forward 
coarse sand, would be sufficiently powerful to carry forward 
a few rounded pebbles, by rolling them over the irregular 
surface of the sand, but not sufficiently powerful to transport 
them over the much more irregular surface of a deposit 
consisting altogether of similar pebbles. On the contrary, 
