344 POPULAR SCIENCE MONTHLY. 



lectively, 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 postulated 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 accretion bore some 

 definite ratio to the present 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 con- 

 fidence 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 thickness 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 represent, foot for foot, 

 greater amounts of time than the coarser sediments gathered by strong 

 currents; and studying the formations with regard to both thickness 

 and texture, geologists have made out what are called time ratios — series 

 of numbers expressing 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 satisfactory determina- 

 tion shall have been made of the duration of any one period. 



Eeade has varied the sedimentary method by restricting attention to 

 the limestones, which have the peculiarity that their material is carried 

 from the land in solution: and it is a poijrt in favor of this procedure 

 that the dissolved burdens of rivers are more easily measured than their 

 burdens of clay and sand. 



An independent system of time ratios has been founded on the prin- 

 ciple of the evolution of life. Not all formations are equally supplied 

 with fossils, but some of them contain voluminous records of contem- 

 porary 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 yet been found possible to 

 make approximate estimates, and these in the main lend support to the 

 time ratios founded on sedimentation. They bring aid also at a point 

 where the sedimentary data are weak, for the earliest formations arc 

 hard to classify and measure. It is true that these same formations are 

 almost barren of fossils, but biologic inference does not therefore stop. 



