XllV PKOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



rent at any place depend on the 'physical history of the locality — a 

 function of local conditions. In other words, the agreements depend 

 on concurrence of time ; the differences on disagreeing local con- 

 ditions. 



The correspondence in time here indicated by the term contem- 

 poraneity may be understood as having only so much of definiteness 

 as belongs to the deposition of a stratified rock. Whatever pheno- 

 mena occurred within the limits of the duration marked by the 

 beginning and ending of such a deposit may be reckoned as of one 

 period, though they may not all be assignable to one precise or 

 momentary epoch. There are good reasons for doubting the exact 

 epochal correspondence of distant parts of the same continuous sedi- 

 mentary deposits (as, for example, in beds of obliquely laminated 

 oolite and millstone-grit, in which the successive deposition of parts 

 of the bed by currents flowing in ascertained directions is certainly 

 traced), but none which forbid our using these deposits to mark a 

 period having a certain place in a series of periods*. 



Geology has thus obtained a true scale of relative time on which 

 to register all the events in the earth's history which fall within the 

 wide compass of her inquiry. Starting with a scale of strata exactly 

 determined, and rendered as perfect as possible in one basin, we 

 acquire a general series or scale of deposits, in the order of their 

 deposition, suited to a certain area of the ancient bed of the sea. 

 By examination of these deposits, we find in each of them fossil 

 marks of an intelligible kind, by which the place of any given step 

 in the series can be determined for the area in question. By com- 

 parison of one large basin of strata with another, it is found that 

 the fossils taken in allied groups have larger ranges than the mi- 

 neral or structural characters of the strata, and indicate that the 

 basins were occasionally, or frequently, in communication, so that 

 some corresponding forms of life are found to occur in both, in 

 several or all of the deposits, and in the same order of succession. 



By thus comparing basin with basin, it is found that hardly one of 

 the stratified systems of rocks was so insulated by the circumstances 

 of its formation as to show no conformity with another in the se- 

 quence of ancient life, however different may be the appearance and 

 however unequal the completeness of the series of the groups of 

 strata. If there were any seas entirely separated (as now we see 

 the Caspian and other seas), the separation of them must have taken 

 place at some definite period, previous to which they were influenced 

 more or less by the general order of life in the other neighbouring 

 parts of the ocean. It may be taken as a general result of all this 

 inquiry, that there is but one general series of life represented in a 

 fossil state, — each term of this series corresponding to a geological 

 period, and, taken in a large sense, preserving one main or general 

 character, amidst many local variations, over all the areas yet inves- 

 tigated. 



* Mr. Godwin-Austen has bestowed much attention on this subject. See also, 

 in my Treatise on Geology in Cab. Cycl. vol. ii. chap, vi., a discussion on this 

 subject, and a diagram in illustration. 



