512 REPORT — 1886. 



the amount of sediment carried down by rivers, it appears that it takes from 1,000 

 to 6,000 years to remove one foot of rock from the general surface of a river 

 basin. 



From a consideration of the denuding power of rivers, and a measurement of 

 the thickness of stratified rock, Phillips has made an estimate of the period of time 

 comprised in geological history, and finds that, from stratigraphical evidence alone, 

 we may regard the antiquity of life on the earth as being possibly between 38 and 

 96 millions of years. ^ 



Now while we should perhaps be wrong to pay much attention to these figures, 

 yet at least we gain some insight into the order of magnitude of the periods with 

 which we have to deal, and we may feel confident that a million years is not an 

 infinitesimal fraction of the whole of geological time. 



It is hardly to be hoped, however, that we shall ever attain to any very accurate 

 knowledge of the geological time scale from this kind of argument. 



But there is another theory which is precise in its estimate, and which, if 

 acceptable from other points of view, will furnish exactly what is requisite. Mr. 

 Croll claims to prove that great changes of climate must be brought about by 

 astronomical events of which the dates are known or ascertainable.^ The pertur- 

 bation of the planets causes a secular variability in the eccentricity of the earth's 

 orbit, and we are able confidently to compute the eccentricity for many thousands 

 of years forward and backward from to-day, although it appears that, in the 

 opinion of Newcomb and Adams, no great reliance can be placed on the values 

 deduced from the formulae at dates so remote as those of which Mr. Croll speaks. 

 According to Mr. Croll, when the eccentricity of the earth's orbit is at its 

 maximum, that hemisphere which has its winter in apheUon would undergo a 

 o-lacial period. Now, as the date of great eccentricity is ascertainable, this would 

 explain the great ice-age and give us its date. 



The theory has met with a cordial acceptance on many sides, probably to a great 

 extent from the charm of the complete answer it aifords to one of the great riddles 

 of geology. 



Adequate criticism of Mr. Croll's views is a matter of great difficulty on account 

 of the diversity of causes which are said to co-operate in the glaciation. In the 

 case of an effect arising from a number of causes, each of which contributes its 

 share, it is obvious that if the amount of each cause and of each effect ia largely 

 conjectural the uncertainty of the total result is by no means to be measured by 

 the uncertainty of each item, but is enormously augmented. Without going far 

 into details it may be said that these various concurrent causes result in one 

 fundamental proposition with regard to climate, which must be regarded as the key- 

 stone of the whole argument. That proposition amounts to this — that climate is 

 unstable. _ ... 



Mr. Croll holds that the various causes of change of climate operate inter se in 

 such a way as to augment their several efliciencies. Thus the trade-winds are 

 driven by the difference of temperature between the frigid and torrid zones, and if 

 from the astronomical cause the N. hemisphere becomes cooler the trade-winds on 

 that hemisphere encroach on those of the other, and the part of the warm oceanic 

 current, which formerly flowed into the cold north zone, will be diverted into the 

 S. hemisphere. Thus the cold of the N. hemisphere is augmented, and this in its 

 turn displaces the trade-winds further, and this again acts on the ocean-currents, 

 and so on ; and this is neither more nor less than instability. 



But if climate be unstable, and if from some of those temporary causes, for 

 which no reasons can as yet be assigned, there occurs a short period of cold, then 

 surely some even infinitesimal portion of the second link in the chain of causation 

 must exist : and this should proceed as in the first case to augment the departure 

 from the original condition, and the climate must change. 



In a matter so complex as the weather, it is at least possible that there should 

 be instability when the cause of disturbance is astronomical, whilst there is stability 



' Phillips, Life on the Earth. Kede Lecture, 1860, p. 119. 

 '^ Climate and Time. 



