MOVEMENT IN THE CRUST OF THE EARTH 5 



have been brought into this condition by intermittent convulsions 

 often exhibited in earthquakes. 



Having contemplated the lithosphere as a body moving in 

 upheaval and subsidence, and shown what is about the maximum 

 and minimum of these oscillations and their paroxysmal charac- 

 ter, we are prepared to consider the structure of this crust. 



In all geological ages volcanic eruptions have occurred and 

 rocky material from the depths has been brought to the surface. 

 Such appearances of lava at the surface have been very common 

 in human history, and they appear to have been just as common 

 in all the geological ages revealed by science. Lavas vary in 

 chemical and mineraloarical constitution, but this variation is 

 within narrow limits. All of the mineral substances known to 

 mankind appear, but are intimately mixed as minute ingredients. 

 Lavas, therefore, are intimate mixtures of many substances, the 

 average of which falls within narrow limits. It would appear 

 from our present knowledge that the primordial surface of 

 the earth was cooled lava and that lava has been erupted from 

 time to time through all of the great geological ages. 



Upon these cooled surfaces a new crust of rocks from below 

 and rocks from above appears to have been spread. Wind waves 

 and tidal waves are forever beating the lands and undermining 

 the cliffs and distributing the materials beneath the sea. Then 

 atmospheric agencies disintegrate the rocks and the rains wash 

 the sands into the streams which carry them into the lakes and 

 into the sea. By many cognate processes the lands are worn 

 down and the sea bottoms built up ; the amount of detritus thus 

 accumulated in zones about the meandering shores is great, so 

 that in regions of maximum activity formations are accumulated 

 thousands of feet in thickness. 



The winds contribute to the material which falls into the 

 sea; plant life also furnishes its quota; accumulations of vege- 

 tation are ultimately consolidated among the formations as beds 

 of coal ; and animal life adds to the marine formations, for 

 corals, shells, and bones are all brought to be buried in the 

 sand, and often extensive formations of calcareous matter are 



