OT  — — ———— ———— 
TRANSACTIONS OF SECTION C, 729 
of the subject is not without importance in relation to the denudation not only of 
the existing terrestrial surface but of the lands of former periods, for there is 
evidence of more than one arid epoch in geological history. Here, again, a dili- 
gent examination of ancient buildings and monuments might afford some, at least, 
of the required data. Insucha couniry as Egypt, for instance, it might eventually 
be possible to determine from a large series of observations what has been the 
average rate of surface-disintegration of the various kinds of stone employed in 
human constructions that have been freely exposed to the air for several thousand 
ears. 
f Closely linked with the question of denudation is that of the Deposition of the 
material worn away from the surface of the land. The total amount of sediment 
laid down must equal the amount of material abstracted, save in so far as the 
soluble portions of that material are retained in solution in the sea. But we have 
still much to learn as to the conditions, and especially as to the rate, of sedimenta- 
tion. Nor does there appear to be much hope of any considerable increase to our 
knowledge until the subject is taken up in earnest as one demanding and justifying 
a prolonged series of well-planned and carefully executed observations. We have 
yet to discover the different rates of deposit, under the varying conditions in which 
it is carried on in lakes, estuaries, and the sea, What, for instance, would be a 
fair average for the rate at which the lakes of each country of Europe are now 
being silted up? If this rate were ascertained, and if the amount of material 
already deposited in these basins were determined, we should be in possession of 
data for estimating not only the probable time when the lakes will disappear, but 
also the approximate date at which they came into existence. 
But it is not merely in regard to epigene changes that further more extended 
and concerted observation is needed. Even among Subterranean movements there 
are some which might be watched and recorded with far more care and continuity 
than have ever been attempted. The researches of Professor George Darwin and 
others have shown how constant are the tremors, minute but measurable, to which 
the crust of the earth is subject.1_ Do these phenomena indicate displacements of 
the crust, and, if so, what in the lapse of a century is their cumulative effect on 
the surface of the land ? 
More momentous in their consequences are the disturbances which traverse 
Mountain-chains and find their most violent expression in shocks of Earthquake. 
‘The effects of such shocks have been studied and recorded in many parts of the 
world, but their cause is still little understood. Are the disturbances due to a 
continuation of the same operation which at first gave birth to the mountains? 
Should they be regarded as symptoms of growth or of collapse? Are they accom- 
panied with even the slightest amount of elevation or depression? We cannot 
tell. But these questions are probably susceptible of some more or less definite 
answer. It might be possible, for instance, to determine with extreme precision 
the heights above a given datum of various fixed points along such a chain as the 
Alps, and by a series of minutely accurate measurements to detect any upward or 
downward deviation from these heights. It is quite conceivable that throughout 
the whole historical period some deviation of this kind has been going on, though 
so slowly, or by such slight increments at each period of renewal, as to escape 
ordinary observation. We might thus learn whether, after an Alpine earthquake, 
an appreciable difference of level is anywhere discoverable, whether the Alps as 
a great mountain-chain are still growing or are now subsiding, and we might be 
able to ascertain the rate of the movement. Although changes of this nature may 
have been too slight during human experience to be ordinarily appreciable, their 
very insignificance seems to me to supply a strong reason why they should be 
sought for and carefully measured. ‘hey would not tell us, indeed, whether a 
mountain-chain was called into being in one gigantic convulsion, or was raised at 
wide intervals by successive uplifts, or was slowly elevated by one prolonged and 
continuous movement. But they might furnish us with suggestive information as 
to the rate at which upheaval or depression of the terrestrial crust is now 
going on, 
* Report Brit. Assoc, 1882, p. 95. 
