CHAP, xiv The Record of the Rocks 263 



the work in a given time was once much greater than now, 

 as the Catastrophists maintain. Reasoning from the rate 

 of cooling of lava, Sir William Thomson estimated that 

 living creatures such as now exist could not have inhabited 

 the Earth more than 100,000,000 years ago ; and Professor 

 Tait, calculating from the rate at which the Earth is losing 

 heat ( 291) and its present temperature, concludes that 

 20,000,000 years is more nearly the truth, while even 

 10,000,000 years may include the whole range of possible 

 life on the globe. 



342. Reading the Rock Story. If exactly the same 

 areas of the Earth's surface were always subject either to 

 elevation or depression, we could not discover from the 

 rocks laid bare on the surface any record of the process 

 of their formation. The sedimentary rocks would remain 

 in the subsiding hollows, the older layers being successively 

 covered by newer ones. But it happens that the margins 

 of the world ridges on which sediment is deposited are sub- 

 ject to frequent elevation and depression ( 303), and the 

 sedimentary rocks which are exposed bear traces of these 

 changes which it is the special study of geologists to inter- 

 pret. Where rocks are very much crumpled and folded, it 

 often happens that the strata have been inverted, the bottom 

 beds of a series having been folded back upon the upper 

 beds. When a stratum occurs resting on a different sort 

 of rock, which dips in a different direction or bears signs 

 of ancient erosion, the two are said to be unconformable. 

 This structure is clearly indicative of some time having 

 elapsed since the formation of the older series, and 

 before the accumulation of the overlymg younger beds. 

 The stratified rocks are like the sheets of an unbound 

 book, some of which have been printed over a second time 

 with a later part of the work ; many have been crumpled, 

 torn, and rubbed so that they are illegible ; the numbering 

 of all the pages except the last one has been destroyed, 

 and there are evidently places where several pages together 

 have been dropped out. By reading the legible portions of 

 such a book one could find hints of the development of 

 events if the mutilated work were a history, or of the unfold- 



