SCIENTIFIC WRITINGS. 405 



that the segregation of our atmosphere and our seas 

 from the terrestrial mass was the work of many geo- 

 logical periods and is not yet completed, as the still 

 active geysers and hot springs testify. We shall be 

 compelled to assume a "Geyser period" as a special 

 geological period, which followed the formation of the 

 solid crust, and in which volcanoes and geysers ejected 

 at innumerable places of the solidified surface the 

 specifically lighter masses, especially water and air, 

 and with the help of the varying currents of the sea 

 formed by them deposited the stratified sediments. The 

 assumption too of the upheaval of mountains by internal 

 pressure does not agree with the assumption of a molten 

 or gaseous interior, on which the solid crust floats. 



o 7 



They can only be tangential forces, which have elevated 

 mountains and are still elevating parts of the earth's 

 surface. These tangential forces are supplied by pro- 

 gressive cooling of the interior, since the vault, formed 

 by the solid covering of the earth, would collapse 

 through gravitation, if the vanished fluid interior no 

 longer sufficiently supported it. The phenomenon of 

 volcanic eruptions does not necessitate the hypothesis 

 of an internal pressure, which is stronger than corre- 

 sponds to the weight of the solid crust. When we con- 

 sider that the more recently cooled layers of solid rock 

 must, with progressive refrigeration, be liable to rents, 

 which we feel at the surface as earthquakes, it is clear 

 that such rents may affect also the contiguous cooler 

 crust, already frequently ruptured in former geological 

 periods, and thereby bring about direct communications 



