TRANSACTIONS OF THE SECTIONS. 61 
| stone. Professor Hitchcock then proposes for such footmarks the 
following classification. Class, Ichnites : 
= 1st Family—Tetrapodichnites. 
2nd Family—Sauroidichnites. 
3rd Family—Ornithichnites. 
In the second letter, dated May 9, 1837, Professor Hitchcock an- 
nounces his having forwarded to Mr. Clarke between thirty-five 
and forty specimens (some of the rock itself, and others casts) of the 
footmarks. He states the number of species found since his first pub- 
lication to be fifteen, making the total number 22, Mr. Clarke then 
exhibited to the Section several of the specimens. 
On the Nature and Origin of the various kinds of Transported Gravel, 
occurring in England. By Hucu E. Srrickranp, F.G.S. 
On examining the masses of drifted materials, which, in patches of 
gravel, sand, or clay, cover a considerable portion of the surface of the 
island, Mr. Strickland perceived remarkable distinctions of character. 
Taking the varieties of rocks present as a clew to the direction of the 
forces which have moved them along, it appeared that in some cases 
the pebbles and boulders were derived from the immediate vicinity ; 
in others, that they had probably travelled many hundreds of miles. 
The beds were sometimes wholly unstratified, and at other times 
finely laminated, indicating a violent or tranquil state of the transport- 
ing medium. Some varieties of drift occupy the summits of hills, and 
are independent of the present configuration of the surface ; whilst 
others occur on the sides or bottoms of valleys, having a constant 
relation to the present lines of drainage. And again, some gravel 
beds contain remains of mammalia and lacustrine mollusca,—others 
contain only marine remains; and a large portion appears to be 
destitute of organic remains. Such varied results seem to indicate a 
variety of causes, distinct in kind, and operating at separate epochs ; 
but as all these varieties of detritus are unconformable to the rocks 
on which they rest, and from lying in detached portions are seldom 
brought in contact with each other, it is very difficult to determine 
their respective ages, or to establish the precise number of distinct 
epochs at which they may have been formed. Evidence, however, of 
two periods may be clearly obtained ; and Mr. Strickland proposes to 
call the matter deposited in the first period a marine drift, being the 
result of submarine currents at a time when the central portions of 
England were under the ocean; and that deposited in the second, 
fluviatile drift, having apparently been deposited by ancient rivers 
(or river-lakes) at a time when the whole or a great part of England 
had become dry land. The gravel which covers the midland counties, 
from Cheshire to Gloucestershire, has resulted (as proved by Mr. 
Murchison from the evidence of marine shells) from a marine current 
flowing from the North, between the oolitic hills of England, and the 
