A CENTURY OF GEOLOGY. 277 



lighter as the mountain region is higher than the Mississippi husin 

 region. 



Now, so sensitive is the earth to changes of gravit}^ that, given time 

 enough, it responds to increase or decrease of pressure over large 

 areas by corresponding subsidence or elevation. Hence, all places 

 where great accumulations of sediment are going on are sinking 

 under the increased weight, and, contrarily, all places where excessive 

 erosion is going on, as, for example, on high plateaus and great 

 mountain ranges, are rising by relief of pressure. 



This principle of isostasy is undoubtedly a valuable one, which must 

 be borne in mind in all our reasonings on crust movements, although 

 its importance has been exaggerated by some enthusiastic supporters. 

 Its greatest importance is not as a cause initiating crust movements 

 or determining the features of the earth, but rather as conditioning 

 and modifying the results produced by other causes. The idea 

 belongs wholly to the latter half of the present century. Commenc- 

 ing about 1840, it has grown in clearness and importance to the 

 present time. 



THE AGE OF THE EARTH. 



Until almost the beginning of the present century the general belief 

 in all Christian countries was that not only the earth and man, but the 

 whole cosmos, began to exist about six thousand to seven thousand years 

 ago; furthermore, that all was made at once without natural process, 

 and have remained sulxstantially unchanged ever since. This is the 

 old doctrine of the supernatural origin and substantial permanency of 

 the earth and its features. Among intelligent and especially scientific 

 men this doctrine, even in the eighteenth century, began to be ques- 

 tioned, although not pu])licly; for in 1751 Buffon was compelled by 

 the Sorbonne to retract certain views concerning the age of the earth, 

 published in his Natural History in 1749.^ Remnants of the old belief 

 lingered even into the early part of the present century, and may even 

 yet be found hiding away in some of the remote corners of civilized 

 countries. But with the' birth of geology, and especially through the 

 work of Hutton in Scotland, Cuvier in France, and William Smith ui 

 England, the nuich greater— the inconceivably great— an tiiiuity of the 

 earth and the origin of its present forms, by gradual changes which 

 are still going on, was generally acknowledged. Indeed, as already 

 said, thisls the fundamental idea of geology, without which it could 



not exist as a science. 



Geologv has its own measures of time— in eras, periods, epochs, 

 ages, etc.-but it is natural and right that we should desire more accu- 

 rate estimates by familiar standards. How old, then, is the earth 

 especially the inhabited earth, in years? Geologistsjuive^itteinpted 



1 Lyell's Priuciples of Geology, eighth edition, p. 41. 



