90 The Outline of Science 



the salts of the sea, for all these have been dissolved out of the 

 rocks since rain began to fall on the earth. Dividing the total 

 amount of saline matter by what is contributed every year in 

 modern times, we get about a hundred million years as the age of 

 the sea. But as the present rate of salt-accumulation is probably 

 much greater than it was during many of the geological periods, 

 the prodigious age just mentioned is in all likelihood far below the 

 mark. Another method is to calculate how long it would take to 

 form the sedimentary rocks, like sandstones and mudstones, 

 which have a total thickness of over fifty miles, though the local 

 thickness is rarely over a mile. As most of the materials have 

 come from the weathering of the earth's crust, and as the annual 

 amount of weathering now going on can be estimated, the time re- 

 quired for the formation of the sedimentary rocks of the world 

 can be approximately calculated. There are some other ways 

 of trying to tell the earth's age and the length of the successive 

 periods, but no certainty has been reached. 



The eras marked on the table (page 92) as before the Cam- 

 brian correspond to about thirty-two miles of thickness of strata ; 

 and all the subsequent eras with fossil-bearing rocks to a thickness 

 of about twenty-one miles in itself an astounding fact. Perhaps 

 thirty million years must be allotted to the Pre-Cambrian eras, 

 eighteen to the Palaeozoic, nine to the Mesozoic, three to the 

 Cenozoic, making a grand total of sixty millions. 



The Establishment of Invertebrate Stocks 



It is an astounding fact that at least half of geological time 

 (the Archeozoic and Proterozoic eras) passed before there were 

 living creatures with parts sufficiently hard to form fossils. In 

 the latter part of the Proterozoic era there are traces of one-celled 

 marine animals (Radiolarians) with shells of flint, and of 

 worms that wallowed in the primal mud. It is plain that as re- 

 gards the most primitive creatures the rock record tells us 

 little. 



