STRATIFIED ROCKS. 



7S 



cease to receive deposits, and would be eroded away by the 

 action of the waves and partly redistributed over the other 

 deposits, while the one farthest out (3) would receive first the 

 finer deposits (a"), then still coarser ("), and finally the shore 

 conditions would prevail and their appropriate sediments 

 would be deposited (<:"). The following would result : 



West. 



East. 



FIG. 4. Three sets of deposits formed under the same conditions as those of Fig. 3, except that 

 the land was gradually rising during the accumulation of sediments. In both figures the 

 coarser sediments are represented by open dots, the sands by fine dots, the coarse muds by 

 heavy horizontal lines, the finer muds by similar finer lines. 



There would be a reversal in the order of the sediments ; 

 also a change in the relative thickness of the three sections ; 

 and number 3 would be the thicker section. Although gravel 

 might appear at the top of each section, it would represent a 

 later period in section 3 than in section I, and all the period 

 represented by b" and c" of the third section would be repre- 

 sented in the first section by an hiatus or line of erosion. It 

 is essential to assume that such oscillations, upward or down- 

 ward, were taking place constantly during the accumulation 

 of the sedimentary deposits now called stratified rocks, and 

 the above analysis exhibits the nature of the perplexities 

 which must arise in a precise study of the relations of the for- 

 mations of different regions to each other. 



Characteristic Fossils. In a general way fossils are charac- 

 teristic of the age of the systems, but actually the systems 

 represent great lengths of even geological time ; and in many 

 cases this time is long enough to include the beginning, the 

 luxuriant abundance, and the extinction of a whole genus 

 or a family of organisms. Such generic groups have had 

 their stage of beginning, have spread about the earth, and 

 during their distribution and adaptation to the various con- 

 ditions of environment have become specifically modified, so 

 that each of the systems is marked by the presence of cer- 



