488 



NATURE 



[September 13, 1900 



support can lead to sudden fracture and collapse ; hence the 

 comparatively superficial orifjin of earthquakes. 



Given a sufficiently large coefficient of expansion — and there 

 is much to suggest its existence [vide, p. 483) — and all the 

 phenomena of mountain ranges become explicable : they begin 

 to present an appearance that invites mathematical treatment ; 

 they inspire us with the hope that from a knowledge of the 

 height and dimensions of a continent and its relations to the 

 bordering ocean we may be able to predict when and where a 

 mountain chain should arise, and the theory which explains 

 them promises to guide us to an interpretation of those world- 

 wide unconformities which Suess can only account for by a 

 transgression of the sea. Finally it relieves us of the difficulty 

 presented by mountain formation in regard to the estimated 

 duration of geological time. 



Influence of Variations in the Eccentricity of the Earth's Orbit. 

 This may perhaps be the place to notice a highly interesting 

 speculation which we owe to Prof. Blytt, who has attempted to 

 establish a connection between periods of readjustment of the 

 earth's crust and variations in the eccentricity of the earth's 

 orbit. Without entering into any discussion of Prof. Blytt's 

 methods, we may offer a comparison of his results with those 

 that follow from our rough estimate of one foot of sediment 

 accumulated in a century. 



Table showing the Time that has elapsed since the Beginning of 

 the Systems in the first column, as reckoned from Thickness 

 of Sediment in the second column, and by Prof Blytt in the 

 third .— 



Years Years 



Eocene ... 4,200,000 ... 3,250,000 



Oligocene ... 3,000,000 ... 1,810,000 



Miocene ... 1,800,000 ... 1,160,000 



Pliocene ... 900,000 ... 700,000 



Pleistocene ... 400,000 ... 350,000 



It is now time to return to the task, too long postponed, of 

 discussing the data from which we have been led to conclude 

 that a probable rate at which sediments have accumulated in 

 places where they attain their maximum thickness is one foot 

 per century. 



Rate of Deposition of Sediment. 



We owe to Sir Archibald Geikie a most instructive method 

 ■of estimating the existing rate at which our continents and 

 islands are being washed into the sea by the action of rain and 

 ■rivers : by this we find that the present land surface is being 

 ■reduced in height to the extent on an average of 1/2400 foot 

 yearly (according to Prof. Penck 1/3600 foot). If the material 

 removed from the land were uniformly distributed over an area 

 €qual to that from which it had been derived, it would form a 

 layer of rock 1/2400 foot thick yearly, i.e. the rates of denuda- 

 tion and deposition would be identical. But the two areas, 

 that of denudation and that of deposition, are seldom or never 

 equal, the latter as a rule being much the smaller. Thus the 

 area of that part of North America which drains into the Gulf of 

 Mexico measures 1,800,000 square miles ; the area over which 

 its sediments are deposited is, so far as I can gather from Prof. 

 Agassiz' statements, less than 180,000 square miles ; while Mr. 

 McGee estimates it at only 100,000 square miles. Using the 

 largest number, the area of deposition is found to measure one- 

 tenth the area of denudation ; the average rate of deposition 

 -will therefore be ten times as great as the rate of denudation, 

 ■or -2^5^ foot may be supposed to be uniformly distributed over 

 the area of sedimentation in the course of a year. But the 

 thickness by which we have measured the strata of our geo- 

 ilogical systems is not an average but a maximum thickness ; we 

 have therefore to obtain an estimate of the maximum rate of 

 ■deposition. If we assume the deposited sediments to be 

 arranged somewhat after the fashion of a wedge with the thin 

 end seawards, then twice the average would give us the maxi- 

 mum rate of deposition ; this would be one foot in 120 years. 

 But the sheets of deposited sediment are not merely thicker 

 towards the land, thinner towards the sea, they also increase in 

 thickness towards the rivers in which they have their source, so 

 that a very obtuse-angled cone, or, better, the down-turned 

 ,t)owl of a spoon, would more nearly represent their form. This 

 iform tends to disappear under the action of waves and currents, 

 .but a limit is set to this disturbing influence by the subsidence 

 ■which marks the region opposite the mouth of a large river. By 



NO, 161 I, VOL. 62] 



this the strata are gradually let downwards, so that they come 

 to assume the form of the bowl of a spoon turned upwards. 

 Thus a further correction is necessary if we are to arrive at a 

 fair estimate of the maximum rate of deposition. Considering 

 the very rapid rate at which our ancient systems diminish in 

 thickness when traced in all directions from the localities where 

 they attain their maximum, it would appear that this correction 

 must be a large one. If we reduce our already corrected esti- 

 mate by one-fifth, we arrive at a rate of one foot of sediment 

 deposited in a century. 



No doubt this value is often exceeded ; thus in the case of 

 the Mississippi River the bar of the south-west pass advanced 

 between the years 1838 and 1874 a distance of over 2 miles, 

 covering an area 2 '2 miles in width with a deposit of sediment 

 80 feet in thickness; outside the bar, where the sea is 250 feet 

 in depth, sediment accumulates, according to Messrs. Humphreys 

 and Abbot, at a rate of 2 feet yearly. It is quite possible, 

 indeed it is very likely, that some of our ancient strata have 

 been formed with corresponding rapidity. No gravel or coarse 

 sand is deposited over the Mississippi delta ; such material is 

 not carried further seawards than New Orleans, Thus the vast 

 sheets of conglomerate and sandstone which contribute so 

 largely to some of our ancient systems, such as the Cambrian, 

 Old Red Sandstone, Millstone Grit, and Coal Mea.sures, must 

 have accumulated under very different conditions, conditions for 

 which it is not easy to find a parallel ; but in any case these 

 deposits afford evidence of very rapid accumulation. 



These considerations will not tempt us, however, to modify our 

 estimate of one foot in a century ; for though in some cases this 

 rate may have been exceeded, in others it may not have been 

 nearly attained. 



Closely connected with the rate of deposition is that of the 

 changing level of land and sea ; in some cases, as in the 

 Wealden delta, subsidence and deposition appear to have pro- 

 ceeded with equal steps, so that we might regard them as trans- 

 posable terms. It would therefore prove of great assistance if 

 we could determine the average rate at which movements of the 

 ground are proceeding ; it might naturally be expected that the 

 accurate records kept by tidal gauges in various parts of the 

 world would afford us some information on this subject ; and 

 no doubt they would, were it not for the singular misbehaviour 

 of the sea, which does not maintain a constant level, its fluctua- 

 tions being due, according to Prof Darwin, to the irregular 

 melting of ice in the polar regions. Of more immediate 

 application are the results of Herr L. Holmstrom's observations 

 in Scandinavia, which prove an average rise of the peninsula at 

 the rate of 3 feet in a century to be still in progress ; and Mr. 

 G. K. Gilbert's measurements in the great Lake district of 

 North America, which indicate a tilting of the continent at the 

 rate of 3 inches per hundred miles per century. But while 

 measurements like these may furnish us with some notion of the 

 sort of speed of these changes, they are not sufficient even to 

 suggest an average ; for this we must be content to wait till 

 sufficient tidal observations have accumulated, and the disturbing 

 effect of the inconstancy of the sea-level is eliminated. 



It may be objected that in framing our estimate we have 

 taken into account mechanical sediments only, and ignored 

 others of equal importance, such as limestone and coal. With 

 regard to limestone, its thickness in regions where systems 

 attain their maximum may be taken as negligible ; nor is the 

 formation of limestone necessarily a slow process. The success- 

 ful experiments of Dr. Allan, cited by Darwin, prove that reef- 

 building corals may grow at the astonishing rate of six feet in 

 height per annum. 



In respect of coal there is much to suggest that its growth 

 was rapid. The carboniferous period well deserves its name, 

 for never before, never since, have Carbonaceous deposits ac- 

 cumulated to such a remarkable thickness or over such wide 

 areas of the earth's surface. The explanation is doubtless partly 

 to be found in favourable climatal conditions, but also, I think, 

 in the youthful energy of a new and overmastering type of vege- 

 tation, which then for the first time acquired the dominion of 

 the land. If we turn to our modern peat-bogs, the only Car- 

 bonaceous growths available for comparison, we find from data 

 given by Sir A. Geikie that a fairly average rate of increase is 

 six feet in a century, which might perhaps correspond to one 

 foot of coal in the same period. 



The rate of deposition has been taken as uniform through the 

 whole period of time recorded by stratified rocks ; but lest it 

 should be supposed that this involves a tacit admission of 



