6 
conditions.” We make no apology for calling so early, or for 
presently quoting so often the words of Mr. Prestwich. No 
one can follow in a path once trodden by him, without using 
his footprints. He thus admits the futility of present opera- 
tions, and points to the greater agencies of the past. “ River 
action of greater intensity and periodical floods imparting a 
torrential character to the rivers, the consequences of the joint 
operation are obtained.” * 
He refers to his reasoning as that which thus “ brings down 
the larger mammalia to a period subsequent to that when 
the extreme glacial condition prevailed, and closer to our own 
times.” . “ These conditions, taken as a whole, are com- 
patible only with the action of rivers, flowing in the direction 
of the present rivers, and in operation before the existing 
valleys were excavated through the higher plains, of power and 
volume far greater than the present rivers, and dependent upon 
climatal causes distinct from those now prevailing in these 
latitudes. The size, power, and width of the old rivers is 
clearly evinced by the breadth of their channel, and the 
coarseness and mass of their shingle beds ; whilst the volume 
and power of the periodical inundations arc proved by the great 
height to which the flood silt has been carried above the 
ordinary old river levels, — floods which swept down the marsh 
and land shells, together with the remains of animals of the 
adjacent shores, and entombed them either in the coarser 
shingle of the main channel, or else in the finer sediment 
deposited by the subsiding waters in the more sheltered posi- 
tions.” f • . • • “ To estimate the time to which we have to 
carry back the high-level gravels, we have to consider what 
may have been the duration of their accumulation, and that 
of the subsequent excavation of the valleys with the resulting 
low-level gravels. A difficulty here meets us at the onset. 
The accumulation of sand, gravel, and shingle along the course 
of rivers is so irregular (sometimes very rapid, at other times 
slow, — what is done one year being undone another), that we 
are entirely without even the few data by which we are 
approximately guided in ordinary sedimentary strata. The 
thickness of the deposits affords no criterion of the time 
required for their accumulation. They rarely exceed 20 feet, 
and are more frequently not above 10 feet to 12 feet thick. 
It is well known that recent inundations have covered valleys 
with sand and gravel to the depth in places of four, six, or even 
* Philosophical Transactions, part ii., 18C4, p. 250. 
f lb., p. 280 . 
