468 Notices of Memoirs — British Association — 



sedimentation. Nor does there appear to be ranch 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 recoi'ded 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.^ 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 effect 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 slowlj^ 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. They 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, 

 1 Eeport Brit. Assoc, 1882, p, 95. 



