THE ORIGIN AXD DESCENT OF ROCKS. 



395 



e.dge. In the portions underground there were insinuations into thin 

 fissures, on the one hand, and in-thrustings of thick bodies, on the other. 

 Some intrusions entered the upper part of the crust where the rocks 

 were cold and wet, and some were thrust into the deeper portions where 

 the rocks were warmer and less penetrated by water. Sometimes the 

 lava rose rapidly and was httle cooled in passage, sometimes slowly 

 with more coohng en route, and sometimes there were long halts between 

 eruptions, with much opportunity to cool. An almost infinite variety 



Fig. 335. — Fresh lava flow, -^-ith large surface exposure. Holemaumau, Hawaii. 



(Libbey.) 



of conditions is thus presented, and with it a rich field for the study of 

 the modes of solidification. 



In the underground intrusions the additional factor of high pres- 

 sure was also present, and tliis is the third important condition in deter- 

 mining the nature of igneous rocks. 



The three factors, composition, rate of cooling, and degree of pressure, 

 require special consideration. 



Composition of Igneous Rocks. 



All or nearly all the chemical elements known on the earth are found 

 in greater or less amounts in igneous rocks, and in a broad sense are 

 constituents of them. If there are any exceptions, they are most 

 hkely to be found in the rarer elements in the atmosphere. Oxygen, 



