286 OUTLINES OF EVOLUTIONABY BIOLOGY 



of sea-bottom, so that every foot in depth worn off the land is 

 taken to indicate a foot in thickness piled up in new deposits. 

 This hardly seems to be a justifiable assumption, for in many 

 localities sediment derived from a large area must be accu- 

 mulated over a small one, and the increase in thickness of the 

 newly forming rock must therefore take place at a much greater 

 rate than the decrease in thickness of that which is being worn 

 away. 



Professor Sollas, in his presidential address to the Geological 

 Society of London in 1909, gives the total maximum thickness of 

 the sedimentary rocks from the bottom of the Cambrian to the top 

 of the most recent formations as being no less than 253,000 feet. 

 I have accepted this estimate in drawing up the geological time- 

 scale on page 284, in which the supposed thickness of the 

 deposits belonging to each of the main subdivisions of the three 

 great eras is indicated both numerically and by the relative depth 

 of the space allotted to each. 



Professor Sollas, however, estimates a much more rapid rate of 

 accumulation of these deposits as high, indeed, as one foot per 

 century which, even with his own increased estimate of thickness, 

 would give only 25,800,000 years as the total lapse of time since 

 the commencement of the Cambrian epoch. But he also points 

 out that there is good reason for believing that the pre- 

 Cambrian epoch must have been as long as all the succeeding 

 epochs put together, and the total lapse of time since the formation 

 of the stratified rocks (including the pre-Cambrian) commenced 

 might accordingly be estimated at 50,600,000 years. 



We must also, of course, make abundant allowance for the fact 

 that denudation has been going on at all times since the deposition 

 of the stratified rocks commenced, and that the total thickness of 

 these rocks has been enormously reduced thereby, the same 

 material having been used over and over again in rock formation. 



Where opinions differ so much, and where there is so much 

 uncertainty as to the value of the data themselves, it is obvious 

 that any estimate of geological time derived from the thickness 

 of the sedimentary rocks can have only a doubtful value, and that 

 it is highly desirable that such an estimate should be checked 

 by making use of other sources of information. 



Several other methods of estimating the age of the earth have 

 been employed. Perhaps the most satisfactory is that which is 

 especially associated with the name of Professor Joly, and which 



