THE 
GHOLOGICAL MAGAZINE. 
NEW TSERIESS DECADE (ili VOEU xX, 
No. III.—_MARCH, 1893. 
(G)A5v IE CHIEIN | Auda, Yes bane Ess) 
I.—MEASUREMENT OF GEOLOGICAL TIME. 
By T. Metuarp Reape, C.E., F.G.S. 
ANY ways of measuring geological time have been attempted 
3 by various geologists of eminence; but however diverse their 
methods, so far as I know, all finally hinge upon either the rates of 
denudation or accumulation. It is urged as an objection to this that 
in past ages such actions have gone on more rapidly, and therefore 
the calculations based upon present rates are valueless. Physicists 
on the other hand have sought to put a limit to the age of the earth 
much below what geologists generally demand. Reasoning from 
certain data which are necessarily more or less hypothetical, they 
say, that from the thermal condition of the globe at present, it cannot 
be more than from ten to twenty million years since it was at 
a temperature in which life on it would have been impossible. 
Geologists can hardly be blamed if they attach greater weight to 
their own observations and data and to reasoning that is more 
familiar and appears more certain and satisfactory to their minds.? 
It is, however, quite unnecessary for me to elaborate this point, 
as Sir Archibald Geikie has so lately dwelt upon it in his usual 
lucid and telling manner.’ 
It appears to me after long study and various attempts on my own 
part to approximate to the length of geological time that the difficulty 
of getting at a satisfactory system of calculation does not arise from 
the unreliability of assumed rates of denudation, but from the want 
of a principle which will eliminate uncertain phenomena. If our 
modulus is the maximum thickness of all the formations we are 
naturally met with the objection that such a pile of strata nowhere 
occurs in nature. If we take as our gauge the actual thickness in 
any one place it may be reasonably objected that 10 feet of one set 
of strata may chronologically represent 1000 feet or more of another. 
In addition to this, how can the hiatus represented by unconformity 
be estimated? If, again, we start with the average thickness of 
the sedimentary rocks as our principal measure, it is obvious on the 
slightest consideration that this will give a result much below the 
actual time, as the particles of such sedimentary rocks have been 
used up over and over again, and to reach a correct result the number 
* Since this was written Mr. Clarence King’s paper (‘‘ The Age of the Earth,” 
Amer. Journ. of Science, Jan. 1893), proves the final exception to the rule. 
* Presidential Address to the British Association, Edinburgh, 1892. 
DECADE III.—VOL X.—NO. III. a 
