June 29, 1900.] 



SCIENCE. 



1005 



rivers and torn from the cliffs of the shore 

 by waves. After an estimate has been 

 obtained of the total annual sedimentation 

 at the present time, it is necessary to as- 

 sume either that the average rate in past 

 ages has been the same or that it has dif- 

 fered in some definite way. 



At this point the course of procedure 

 divides. The computer may consider the 

 aggregate amount of the sedimentary rocks, 

 irrespective of their subdivisions, or he may 

 consider the thicknesses of the various 

 groups as exhibited in d liferent localities. 

 If he views the rocks collectively, as a total 

 to be divided by the annual increment, his 

 estimate of the total is founded primarily 

 on direct measurements made at many 

 places on the continents, but to the result 

 of such measurements he must add a pos- 

 tulated amount for the rocks concealed by 

 the ocean, and another postulated amount 

 for the material which has been eroded from 

 the land and deposited in the sea more than 

 once. 



If, on the other hand, he views each 

 group of rocks by itself, and takes account 

 of its thickness at some locality where it is 

 well displayed, he must acquire in some 

 way definite conceptions of the rates at 

 which its component layers of sand, clay 

 and limy mud were accumulated, or else he 

 must postulate that its average rate of ac- 

 cretion bore some definite ratio to the pres- 

 ent average rate of sedimentation for the 

 whole ocean. This course is, on the whole, 

 more difficult than the other, but it has 

 yielded certain preliminary factors in which 

 considerable confidence is felt. Whatever 

 may have been the absolute rate of rock 

 building in each locality, it is believed that 

 a group of strata which exhibits great thick- 

 ness in many places must represent more 

 time than a group of similar strata which is 

 everywhere thin, and that clays and marls, 

 settling in quiet waters are likely to repre- 

 sent, foot for foot, greater amounts of time 



than the coarser sediments gathered by 

 strong currents; and studying the forma- 

 tions with regard to both thickness and 

 texture, geologists have made out what are 

 called time ratios, — series of numbers ex- 

 pressing the relative lengths of the different 

 ages, periods and epochs. Such estimates 

 of ratios, when made by different persons, 

 are found to vary much less than do the 

 estimates of absolute time, and they will 

 serve an excellent purpose whenever a satis- 

 factory determination shall have been made 

 of the duration of any one period. 



Reade has varied the sedimentary method 

 by restricting attention to the limestones, 

 which have the peculiarity that their ma- 

 terial is carried from the land in solu- 

 tion ; and it is a point in favor of this pro- 

 cedure that the dissolved burdens of rivers 

 are more easily measured than their bur- 

 dens of clay and sand. 



An independent system of time ratios has 

 been founded on the principle of the evolu- 

 tion of life. I^Tot all formations are equally 

 supplied with fossils, but some of them con- 

 tain voluminous records of contemporary 

 life ; and when account is taken of the 

 amount of change from each full record to 

 the next, the steps of the series are found to 

 be of unequal magnitude. Though there is 

 no method of precisely measuring the steps, 

 even in a comparative way, it has j'et been 

 found possible to make approximate esti- 

 mates, and these in the main lend support 

 to the time ratios founded on sedimentation. 

 They bring aid also at a point where the sed- 

 imentary data are weak, for the earliest for- 

 mations are hard to classify and measure. It 

 is true that these same formations are almost 

 barren of fossils, but biologic inference does 

 not therefore atop. The oldest known fauna, 

 the Eocambrian, does not represent the be- 

 ginnings of life, but a well advanced stage, 

 characterized by development along many 

 divergent lines ; and bj' comparing Eocam- 

 brian life with existing life the paleontolo- 



