Tlie Era of Invertebrates 



amounts to many thousands of feet. Owing to 

 favorable circumstances in the way these rocks 

 are exposed to view it has been possible to 

 measure them much better abroad than in this 

 country; and in Europe, according to W. J. 

 Sollas, the thickness is not far from 94,000 feet 

 — in other words, about 18 miles. The problem 

 is. How long did it take to w^ash away enough 

 of the earth's surface to form, with the aid of 

 the limestone deposited in the sea, this enor- 

 mous amount of rock ? Naturally the problem 

 is very complicated, so that estimates vary from 

 5,000,000 to 30,000,000 years, and 12,000,000 

 may be taken as a conservative estimate. 

 Certainly this seems long enough for many 

 changes to have taken place among plants and 

 animals, but when we think of those that actu- 

 ally did occur it a]3pears none too long. It 

 may give some idea of the abundant life of this 

 era to say that a few years ago the number of 

 species of fossil invertebrates recorded from the 

 paleozoic rocks of North America alone was 

 13,500, w^hile the progress made in our knowl- 

 edge of the past is shown by the fact that in 



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