100 EF’, Chapman—On Sheared Oolitic Limestones. 
tion will be 5280x2x 3000 x38=95,040,000. The time that has 
elapsed since the commencement of the Cambrian is therefore in round 
figures 95 millions of years. 
ConcLUDING REMARKS. 
Although the foregoing computation of post-Archzan time is on a 
different principle to that adopted by Sir Archibald Geikie in his 
Presidential Address to the British Association referred to, it is 
satisfactory to find that the results come out much the same. It has 
been objected to the latter method that deposition goes on much 
more rapidly than denudation, because it is assumed by those who 
hold this opinion that material which is denuded is spread over a 
much smaller area than that from which it was derived, consequently 
it must accumulate more rapidly. Checked by the method adopted 
in this paper it would appear that the earth’s age geologically 
speaking must be, as inferred in the Presidential Address, some- 
where between 100 million and 600 million years.! 
This is a large margin no doubt, but it is an important thing to 
know. Different men may put different values on the three factors, 
bulk of sediment, rate of denudation, and area of denudation; but 
I-think a fair and impartial examination of the reasoning involved 
in this paper will show that the principle of the calculation is sound. 
II.—On Ooxnrric anp oTHER LimEsToNES wiTH SHEARED 
STRUCTURE FROM ILFRACOMBE. 
By Freprerick Cuapman, F.R.M.S. 
(PLATE V.) 
Introduction. 
ie 1886 the author, during a visit to Ilfracombe, obtained some 
specimens of Middle-Devonian limestones which, at the time 
of collecting, he supposed to be formed of Syringopora, on account 
of the granulated weathered surface, and especially since the coral 
named had been recorded from the locality. 
After revisiting the spot in 1890, and making a somewhat 
extensive collection therefrom, principally for the hope of finding 
micro-organisms, this particular rock again attracted his attention 
by the peculiar granulate surface produced by weathering. A 
section of the rock was made which proved it to be a good example 
of a limestone formed of oolitic grains. Subsequently a series of 
specimens showing shearing in all stages was found, and these were 
capable of being traced back to their original source in the rock 
with oolitic structure. 
It is the main purpose of this account of the limestones to point 
out the interesting changes due to dynamical and chemical agencies, 
in the form of shearing, silicification, and the growth of dolomite 
1 Tt must not be forgotten that to arrive at the earth’s age Archeean time has to 
be added to my estimate of 95 million years, which very materially increases the 
margin of geologic time on which we are allowed to draw. 
