198 STRATIFIED OR SEDIMENTARY ROCKS, 



oscillations of land-surfaces and sea-bottoms in past times, similar to 

 those taking place at the present time, we find that in every known 

 local series of strata there exist many and great gaps ; so many and so 

 great that the record may be regarded as only fragmentary. Such gaps 

 are usually indicated by unconformityo It is the task of the geologist, 

 by extensive comparison of rocks in all countries, to fill up these gaps, 

 and make a continuous series. The leaves of the booh of Time are 

 scattered hither and thither over the surface of the earth, and it is the 

 duty of the geologist to gather and arrange them according to their 

 paging. This is done by comparison of rocks of different localities, 

 partly by their lithological character, but principally by the fossils 

 which they contain. 



2. Lithological Character. — At the present time, in our seas and 

 lakes, deposits are forming composed of sand, clay, mud, and lime, of 

 every kind, in different localities. The same has taken place in previ- 

 ous epochs. Sandstones, limestones, and slates, not differing greatly 

 from those forming at the present time, except in degree of consolida- 

 tion, have been formed in every geological period. Lithological char- 

 acter, therefore, is no test of age. In comparing rocks of widely-sepa- 

 rated localities, as, for example, the rocks of different continents, dif- 

 ference of lithological character is no evidence of difference of age, nor 

 similarity of lithological character of any value in determining a geo- 

 logical horizon. But, as deposits are now being formed of a similar 

 character over considerable areas, so also we find strata (the deposits of 

 previous epochs), continuous and unchanged in lithological character, 

 over large tracts of country. Therefore, in contiguous localities, simi- 

 larity of lithological character becomes a very valuable means of iden- 

 tifying strata. If, in two localities not too widely separated, we find a 

 similar rock, e. g., a sandstone of similar grain and color, we conclude that 

 they probably belong to the same age, or are, in fact, the same stratum. 



3. Comparison of Fossils. — This is by far the best, and in widely- 

 separated localities the only, method of determining the age of rocks. 

 The principle of this method is that every geological epoch has its own 

 fauna and flora with many characteristic forms, by which it may be 

 identified everywhere in spite of those slight differences which result 

 from geographical diversity ; and, therefore, similarity of fossils shows 

 similarity of age. There are, however, certain limitations to the appli- 

 cation of this method which must be borne in mind : 



(a.) The lower marine species are much affected by depths and bot- 

 toms, and therefore we should expect that sandstone fossils, limestone 

 fossils, and slate fossils, would differ in species even in the same epoch. 

 Again, in lake and delta deposits, the entombed species would probably 

 be entirely different from those of marine deposits. We must be care- 

 ful, therefore, to compare fossils of rocks formed under similar conditions. 



