394 



GEOLOGY, 



disruptions which the outer part of the crust has suffered. In certain 

 tracts there have been profound fractures, and the formations on one 

 side of these have settled down and on the other side have been pushed 

 up (faulted), so as to expose parts that were once much below the 

 surface. Sometimes also the crust has been folded and crumpled, 

 and the wrinkles thus formed have afterwards been worn away or 

 cut open by deep valleys, and rocks that were once deeply buried 

 have been laid bare. By the revelations made in these and other 

 ways, it . has been learned that at various times in the history of the 

 earth molten matter has been thrust into fissures or intruded between 

 layers of the crust and cooled there, without coming to the surface. 

 Sometimes the lava appears to have forced its way into the rocks, and 

 sometimes to have hfted the upper beds and formed great subterranean 

 layers or tumor-like aggregates, called bathyliths and laccoliths (Fig. 

 334). Such intruded bodies of molten rock, soUdifying under the 

 varying conditions of such subterranean situations, are a fruitful source 



Fig. 334. — Diagram of a laccolith. (Gilbert.) 



of instruction respecting the influence of varying rates and modes of 

 cooHng, as well as of other attendant conditions. 



It will thus be readily seen that the rate of cooling of the various 

 molten rocks must have differed very greatly. In the portions poured 

 out upon the surface there were sometimes narrow streams and thin 

 sheets, giving large exposure in proportion to the mass (Fig. 335), and 

 sometimes thick flows and deep pondings in basins and choked valleys, 

 giving massive bodies with relatively small surface exposure. There 

 were explosions of the lava into minute particles with almost instanta- 

 neous coohng, and there were eruptions beneath the sea the peculiar 

 effects of which are rather matters of inference than of positive knowl- 



