248 PROGRESS OF SCIENCE IN THE CENTURY. 



data as to the rate of cooling of earth, and sun, as to 

 tidal retardation, as to the rate of sedimentation, as to 

 the rate of evolutionary change in organisms, are in 

 varying degrees only approximate, and the age of the 

 earth remains a problem for the twentieth century, 



READING THE ROCK-RECORD. 



We have now grown accustomed to the idea that 

 the strata of the earth's crust form a great library of 

 historical documents relating to the history of our 

 world and its inhabitants, — a library never very com- 

 plete, but, worse than that, disordered, half-burnt, 

 flooded, and buried. 



There are two ways of reading history in this 

 underground library. The nature of the rock, sand- 

 stone or shale, limestone or chert, or otherwise — tells 

 the experienced observer something about the physi- 

 cal conditions of the time when the rock was formed ; 

 and the relation of one stratum or set of strata to 

 another makes it possible to determine the order of 

 succession in time. Yet, on the whole, the decisive 

 evidence as to the physical conditions of the distant 

 age and as to the order of succession in time is 

 afforded by the remains of plants and animals which 

 the rocks contain. 



That fossils furnish the clue which makes it pos- 

 sible to determine the historical order of sequence in 

 the various strata that compose the earth's crust is a 

 familiar fact now; but the realisation of it was a 

 momentous event in the history of geology. And 

 it may be noted that although the study of fossils 

 had begun in the seventeenth century in the in- 

 quiries of Stenson, Hooke, Woodward, and others, al- 

 most no progress was made till the end of the eight- 



