50 EVOLUTION OF THE EARTH 



plete growth, in other words during the closing time of the 

 Formative era and during the Archeozoic era. The differ- 

 ences between the densities of the lands and oceans are held 

 by Barrell to be only skin deep in the outer one-fiftieth of 

 the earth's present diameter, or in the outer shell of about 

 150 miles in thickness. 



On the other hand, it is true that the lands throughout 

 geologic time have repeatedly gone down as well as up, but 

 the sum of their movements has been upward, and for this 

 reason geologists speak of them as the positive areas of the 

 earth's surface. In the same way, the lower-lying fields have 

 also risen locally at times, but their movement has in the 

 aggregate been downward, bringing about the increasingly 

 greater oceanic basins, the negative areas, by far the largest 

 of the earth's surface. We shall see later on that the oceanic 

 fields have become larger at the expense of the lands to 

 accommodate the ever increasing volume of water, and it 

 therefore naturally follows that the continents in former geo- 

 logic times were considerably larger than they are now, pos- 

 sibly even 25 per cent larger. But the newer geology no 

 longer holds to the theory that the oceans and lands have 

 repeatedly changed places; quite the contrary, we agree with 

 Dana that the present positions of the land and water areas 

 have been more or less permanent throughout geologic time. 



Origin of the atmosphere. It was not so long ago that 

 most geologists held with the French astronomer Laplace 

 and with Dana that the earth was originally very hot, and that 

 all of the water of the hydrosphere, the salts of the oceans, 

 and the carbon dioxide of the air and oceans had been parts 

 of the primal gaseous envelope. Not only this, but that the 

 latter also included all the carbon locked up in the rocks, 

 estimated to be 30,000 times greater than the amount now in 

 the atmosphere. Accordingly, as the earth was held to have 

 been cooling throughout its history, it was thought that only 



