SEPTEMBER 21, 1899] 
we cannot but be struck with the predominant vagueness of 
their statements, and with the general absence of such numerical 
data determined by accurate, systematic and prolonged measure- 
ment as would alone furnish a satisfactory basis for computations 
of the rate at which denudation takes place. Some instru- 
mental observations of the greatest value have indeed been 
made, but, for the most part, observations of this kind have 
been too meagre and desultory. 
A little consideration will show that in all branches of the 
investigation of denudation opportunities present themselves on 
every side of testing, by accurate instrumental observation and 
measurement, the rate at which some of the most universal 
processes in the geological végzme of our globe are carried on. 
It has long been a commonplace of geology that the amount 
of the material removed in suspension and solution by rivers 
furnishes a clue to the rate of denudation of the regions drained 
by the rivers. But how unequal in value, and generally how 
insufficient in precision, are the observations on this topic! A 
few rivers have been more or less systematically examined, some 
widely varying results have been obtained from the observ- 
ations, and while enough has been obtained to show the interest 
and importance of the method of research, no adequate supply 
of materials has been gathered for the purposes of accurate 
deduction and generalisation. What we need is a carefully 
organised series of observations carried out on a uniform plan, 
over a sufficient number of years, not for one river only, but for 
all the important rivers of a country, and indeed for alf the 
greater rivers of each continent. We ought to know as accu- 
rately as possible the extent of the drainage-area of each river, | 
the relations of river-discharge to rainfall and to other meteoro- 
logical as well as topographical conditions ; the variation in the 
proportions of mechanical and chemical impurities in the river- 
water according to geological formations, form of the ground, 
season of the year and climate. The whole geological régze 
of each river should be thoroughly studied. The admirable 
report of Messrs. Humphreys and Abbot on the ‘‘ Physics and 
Hydraulics of the Mississippi,’’ published in 1861, might well 
serve asa model for imitation, though these observers neces- 
sarily occupied themselves with some questions which are not 
specially geological and did not enter into others on which, as 
geologists, we should now gladly have further information. 
Again, the action of glaciers has still less been subjected to 
prolonged and systematic observation. The few data already 
obtained are so vague that we may be said to be still entirely 
ignorant of the rate at which glaciers are wearing down their 
channels and contributing to the denudation of the land. 
The whole of this inquiry is eminently suitable for combined 
research. Each stream or glacier, or each well-marked section 
of one, might become the special inquiry of a single observer, 
who would soon develop a paternal interest in his valley and 
vie with his colleagues of other valleys in the fulness and 
accuracy of his records. 
Ner is our information respecting the operations of the sea 
much more precise. Even in an island like Great Britain, 
where the waves and tides effect so much change within the 
space of a human life-time, the estimates of the rate of advance 
or retreat of the shore-line are based for the most part on no 
accurate determinations. It is satisfactory to be able to 
announce that the Council of this Association has formed a 
committee for the purpose of obtaining full and accurate inform- 
ation regarding alterations of our coasts, and that with the 
sanction of the Lords of the Admiralty, the co-operation of the 
coast-guard throughout the three kingdoms has been secured. 
We may therefore hope to be eventually in possession of trust- 
worthy statistics on this interesting subject. 
The disintegration of the surface of the land by the combined 
agency of the subaérial forces of decay is a problem which has 
been much studied, but in regard to whose varying rates of 
advance not much has been definitely ascertained. The 
meteorological conditions under which it takes place differ 
materially according to latitude and climate, and doubtless its 
progress is equally variable. An obvious and useful source of 
information in regard to atmospheric denudations is to be found 
in the decay of the material of buildings of which the time of 
erection is known, and in dated tombstones. Twenty years ago 
I called attention to the rate at which marble gives way in such 
a moist climate as ours, and cited the effects of subaérial waste 
as these can be measured on the. monuments of our graveyards 
and cemeteries (Proc. Roy. Soc. Edin., vol. x. 1879-80, p. 518). 
I would urge upon town geologists, and those in the country 
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| land. 
501 
who have no opportunities of venturing far afield, that they 
may do good service by careful scrutiny of ancient buildings and 
monuments. In the churchyards they will find much to occupy 
and interest them, not, however, like Old Mortality, in repair- 
ing the tombstones, but in tracing the ravages of the weather 
upon them, and in obtaining definite measures of the rate of 
their decay. 
The conditions under which subaérial disintegration is effected 
in arid climates, and the rate of its advance, are still less known, 
seeing that most of our information is derived from the chance 
observations of passing travellers. Yet this branch 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 diligent examination of 
ancient buildings and monuments might afford some, at least, of 
the required data. In sucha country 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 years. 
Closely linked with the question of denudation is that of the 
deposition of the material worn away from the surface of the 
The total amount of sediment laid down must equal the 
amount of material abstracted, save in so far as the soluble por- 
tions 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 sedimentation. 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 justify- 
ing 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 dis- 
appear, 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 Prof. George 
Darwin and others have shown how constant are the tremors, 
minute but measurable, to which the crust of the earth is sub- 
ject (Report Brit. Assoc., 1882, p. 95). 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 accompanied 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 any- 
where 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 care- 
fully measured. They would not tell us, indeed, whether a 
mountain-chain was called into being in one gigantic convulsion, 
