1906.] 



SEE— THE CAUSE OF EARTHQUAKES. 339 



injected with light volcanic materials. Hence the substratum of 

 such a plateau is composed of light materials similar to porous lava 

 and pumice. Only a few mountains, as Orizaba, Popocatapetl, 

 Colima, Jorullo, experienced such violent forces as to come to 

 eruption, but the materials under all of them are the same. The 

 detritus of ashes which the volcanoes eject is nothing but pumice 

 ground to dust by friction and explosive violence. It is obvious 

 that the pumice, ashes and other volcanic materials are not made at 

 the time of ejection, but are always in store, and simply happen to 

 be thrown out of any mountain which experiences eruption. We 

 may in all probability conclude that the plateaus of Tibet, Quito, 

 Caxamarca, Cuzco, Titicaca, and other elevated regions are under- 

 laid by light materials like that thrown from volcanoes. This is 

 certainly true of the great ridges of the Himalayas, Alps, Andes 

 and other mountains which form cordilleras. In fact, the ridges 

 of cordilleras always rest upon a substratum of light volcanic 

 materials, and if they could be exploded from within they would 

 dispense ashes, pumice and scoriae as abundantly as any volcano. 

 The quantity of this light material forced up under the ridges of 

 mountains depends upon the elevation, and breadth, and is thus enor- 

 mous in our highest ranges like the Andes and Himalayas. In this 

 way we may explain many of the anomalies of geodesy, and pen- 

 dulum observations, without any other hypothesis. Is not the ac- 

 cordance of observation with so simple a theory the best proof that 

 the result represents a general law of nature ? 



§ 26. On the theory that large segments of the lithosphere act as 

 units and squeeze those segments zvhich lie between. 



In the new work on " Geology " by Chamberlin and Salisbury, 

 which represents the trend of current geological thought, the authors 

 adopt a subdivision of the earth into large segments which are sup- 

 posed to act as units. The contraction theory is the basis of the 

 reasoning, and part of the discussion is as follows : 



" The downward movements are unquestionably the primary ones, and 

 the horizontal ones are secondary and incidental. The fundamental feature 

 is doubtless central condensation actuated by gravity, and the master move- 

 ments are the sinking of the ocean-basins. The great periodic movements 

 that made mountains and plateaus, and changed the capacity of the ocean- 

 basins, probably started with the sinking of part or all of the ocean-bottoms. 



