Reviews — Prestwich on Valley-Deposits. 25 
water at the time, erosion, and that on no slight scale, must probably 
have been going on; and in the Valley of the Somme there are 
gravels at a higher level and older than those of St. Acheul, which 
may possibly belong to such a period. Indeed, Mr. Prestwich him- 
self refers to some high-level gravels which must have been formed 
before the country became inhabited, and which would therefore 
also be unfossiliferous. 
As to the time required for the excavation of the valleys even 
under the most favourable conditions for it consistent with the ex- 
istence of the animal life, the relics of which are found in the gravels, 
Mr. Prestwich is judiciously cautious in expressing an opinion. We 
cannot do better than quote his own words upon this point :—‘ All 
these phenomena indicate long periods of time. I do not, however, 
feel that we are yet in a position to measure that time, or even to 
make an approximate estimate respecting it. That we must greatly 
extend our present chronology with respect to the first existence of 
man appears inevitable ; but that we should count by hundreds of 
thousands of years is, | am convinced, in the present state of the 
enquiry unsafe and premature. Nevertheless, just as though igno- 
rant of the precise height and size of a mountain-range seen in the 
distance, we need not wait for trigonometrical measurements to feel 
satisfied in our own minds of the magnitude of the distant peaks, so 
with this geological epoch, we see and know enough of it to feel 
how distant it is from our time, and yet we are not in a position at 
present to solve with accuracy the curious and interesting problem 
of its precise age.’ 
As containing some elements for the approximate estimate of the 
duration of time, Mr. Prestwich calls attention to one or two points 
besides the mere changes in the configuration of the surface and the 
alteration which has taken place in the mammalian fauna. Our 
space, however, precludes more than a mere mention of them. One 
of these is the character of the cylindrical or funnel-shaped gravel- 
and sand-pipes which occur in most calcareous rocks, and which are 
in all probability due to the infiltration of water charged with car- 
bonie acid. Several of these pipes occur along the Valley of the 
Somme under conditions which prove that they must have origi- 
nated since the first emergence of the high-level gravels above the 
old river-bed ; and, could a rate of progress be assigned to their 
(rosion, these would give a good gauge of time. One of them, near 
Drucat, must be nearly 100 feet in depth ; and M. Boucher de 
Perthes possesses flint implements reported to have come out of the 
gravel contained in the pipe. On the slope of the valley of the 
Hscardon, close by, is a bed of travertin or calcareous tufa, which 
commences 70 feet above the present level of the stream, and testi- 
fies to a period when the valley had not yet been excavated, and the 
water in the pervious beds of Chalk could find no lower vent. 
Another method suggested bears reference to the probable pertur- 
bations in the increment of heat at different depths in consequence of 
the refrigeration of this part of the surface of the earth at the 
Glacial Period, of which it seems possible some trace might be left 
