CONTAINING FLINT IMPLEMENTS, AND ON THE LOESS. 
303 
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, I am convinced, in the present state of the inquiry, unsafe and 
premature. 
Nevertheless, just as, though ignorant 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. 
Before leaving this subject I would direct attention to one other condition connected 
with this later division of the glacial period, which possibly may eventually afford an 
additional clue towards the solution of this important time-question. Here, again, we 
have not at present all the data we require, but we have enough to show the possibility 
of obtaining from this source some elements for more exact calculations. 
In conducting experiments upon the temperature of the crust of the earth, it is well 
known that, after passing the limits of the line of mean annual temperature, there is a 
gradual increase of l°Fahr. for every 60 feet, nearly, of additional depth. But the rule 
is by no means constant, the rate of increase being subject to fluctuations and variations 
for which no sufficient reason has been assigned. Is it not possible that these dis- 
turbances may arise from differences between the former (glacial) and the present (tem- 
perate) temperature of the place, combined with the variable conductivity of the strata \ 
Let us, for example, take a place like Yakutsk in Siberia, where the ground is per- 
petually frozen to a depth of 382 feet — the depth, therefore, at that place of the line 
of 32°. To reach a heat of 53°, the invariable constant under the Observatory of Paris 
at a depth of 90 feet, we should have to sink at Yakutsk (taking, as a mean, an increase 
of 1° for every 60 feet) to a depth of 382 + (21 X 60), or 1642 feet, before reaching the 
same isothermal plane. If, from any circumstances connected with geological changes, 
we could suppose the mean temperature of Yakutsk to be raised to that of this part of 
Europe, the isothermal plane of 53° would tend to take a vertical range upwards of 
1642 — 90 = 1552 feet. In a perfectly homogeneous mass, and all conditions equal, this 
plane would travel at all parts in equal times, or would move in lines parallel with the 
original position it held ; but as such uniformity over large areas never obtains, and the 
strata which it would have to pass through must differ materially in conductivity of 
heat, it follows that the isothermal planes would, in different places, travel with different 
velocities, and, until adjusted by lapse of time, aberrations in the increment of heat at 
different depths must exist. I apprehend that a very long period of time would also 
elapse before an equilibrium in accordance with the changed mean temperature of the 
place could be established in each successive zone of depth. 
Now if we apply this hypothetical case to these parts of Europe, the question I would 
suggest is if it might not be possible to determine, by calculations founded upon suffi- 
mdccclxiv. 2 T 
