8 /. Clifton Ward — On Geological Time. 



II. — Suggestions as to Geological Timb. 

 By J. Clifton "Ward, F.G.S., of the Geological Survey of England and "Wales. 



BUT a few years since geologists were in the habit of drawing 

 largely from the Bank of time, and the more they drew the 

 greater was the supply they seemed to want, till at last the thought 

 seemed to come quite natural to them that the supply indeed was 

 inexhaustible. Quite lately, however, these time-speculators have 

 had it suggested to them that there is a limit to the supply, and that 

 the countless ages on which they have depended seem to be very 

 probably but some one hundred millions of years, and untold ages 

 to be reckoned by telling numerals. 



Just as, however, it would be doubtless a fault in geologists to 

 look up to mathematicians as infallible in the results at which they 

 arrive regarding geological phenomena, so is it unwise to accept for 

 granted the explanation of like phenomena from a cosmical point of 

 view without first putting it to every possible geological test. Now 

 Mr. Croll has very clearly shown that 240,000 years ago our earth 

 was so situated as regards the eccentricity of its orbit, and the posi- 

 tion of its northern hemisphere during the summer and winter sol- 

 stices, as gradually to bring on a glacial climate, lasting until some 

 80,000 years ago, though having two or more warm periods included 

 within this term of 160,000 years. 



Can any geological facts be brought forward to test this theory ? 



Now it seems to me that there are but two classes of facts which 

 can be of use in such an investigation, viz. : — ■ 



1st. The building up, or wearing away and deposition, of solid 

 material. 



2nd. The rising, or falling, of the earth's crust. 



Under the first class of facts — 1st. Building up. Suppose a Coral- 

 reef 1000 feet in thickness to be formed on a spot where the re- 

 frigerating effects of the glacial climate must have prevented the 

 existence of reef-builders during that epoch, and that such reef pre- 

 sents every appearance of continuity of growth ; then, provided we 

 can form an estimate of the average growth of such a reef under 

 favourable conditions, the least time required may be calculated : for 

 instance, taking one foot per century as the rate of increase in 

 vertical height, a reef 1000 feet in thickness would take at the least 

 100,000 years in forming, and we should conclude that the Grlacial 

 period must have ended not less than 100,000 years ago. 



2nd. Wearing away and deposition. — Sand and clay to a thickness 

 of some 60 feet is found lying upon Grlacial Drift, or surrounding and 

 partly overlapping mounds of such drift, as, for example, in the 

 neighbourhood of York. In the artificial process of warping, each 

 tide not unfrequently deposits about l-12th of an inch of fresh soil. 

 If comparisons could be made between the natural and artificial 

 methods of warping, the difference in area being allowed for in the 

 two cases, some sort of conclusion might be arrived at as to the 

 length of time that the materials for 60 feet of warp took in being 

 denuded from one spot and deposited in another, such a period being 



