22 INTRODUCTION. 



mum, and say that the sum total represents the thickness of 

 our stratified rocks. The varying thicknesses and characters 

 of our formations may be frequently accounted for by con- 

 sidering that the changes in physical conditions did not take 

 place uniformly ; but that a clayey, a sandy, or a calcareous 

 formation may have occupied a longer period in its deposition 

 in one locality than in another, while the succeeding deposit 

 may tell of conditions which locally were of shorter duration. 

 We cannot, however, be exact in our measurements, nor is 

 it possible to calculate with certainty the amount of strata 

 unrepresented in our Islands. Sir A. C. Ramsay, indeed, 

 would lead us to believe that the unrepresented strata were 

 as great as, perhaps greater than, those preserved to us.^ It is 

 true that we know but little of the very earliest or Pre- 

 Cambrian strata ; but between them and the Cambrian rocks 

 there is no evidence of any great break. Locally, there is 

 great unconformity between the Cambrian and Silurian strata 

 of Sedgwick, and there is usually a great break between the 

 Coal Measures and overlying New Red rocks ; locally there 

 are breaks between the Wealden beds and Lower Greensand, 

 and between the Lower Greensand and Gault ; there is a 

 great palaeontological and physical break between the Chalk 

 and the Tertiaries ; and between our Eocene and Pliocene 

 beds there is a break, which is bridged over by the Miocene 

 deposits of other countries. Of course, there are many 

 minor unconformities, but on the whole the sequence of 

 strata in England and Wales gives a fair idea of the succes- 

 sion of changes which the earth's surface has undergone. 



Sir William Thomson has calculated that the sun has 

 probably not illuminated the earth for 100,000,000 years, and 

 almost certainly not for 500,000,000 years. This gives a limit 

 to our estimates of time, which it is desirable not to over- 

 step. The question has naturally arisen whether this esti- 

 mated period of time is sufficient for all geological changes. 

 Taking 100,000 feet as a full allowance for the total thickness 

 of stratified rocks containing traces of life. Professor Huxley 

 has pointed out that, restricting the time to 100,000,000 years, 

 the deposits may be estimated to have taken place at the rate 

 of ToVo of a foot, or •^3- of an inch per annum. And this 

 is a rate which no one can consider too rapid. At the same 

 time, such an estimate is exceedingly vague, for sandstones 

 and limestones would be formed at very different rates. 

 Moreover, the important fact must not be overlooked, that 

 in the very earliest geological periods each bed of sand, clay 



1 Addresses to Geol. Soc. 1S63, 1864; see also T. McK. Hughes, Proc. 

 Cambridge Phil. Soc. iii. 247. 



