14 THE DAWN OF LIFE. 



the sea now deposits, or such as form the common 

 plebeian rocks of everyday plains and hills and coast 

 sections. Being extremely old, however, they have 

 been buried deep in the bowels of the earth under 

 the newer deposits, and hardened by the action of 

 pressure and of heat and heated water. Whether 

 this heat was part of that originally belonging to the 

 earth when a molten mass, and still existing in its 

 interior after aqueous rocks had begun to form on its 

 surface, or whether it is a mere mechanical effect of 

 the intense compression which these rocks have 

 suffered, may be a disputed question; but the ob- 

 servations of Sorby and of Hunt (the former in con- 

 nection with the microscopic structure of rocks, and 

 the latter in connection with the chemical conditions 

 of change) show that no very excessive amount of 

 heat would be required. These observations and those 

 of Daubree indicate that crystallization like that of 

 the Laurentian rocks might take place at a temperature 

 of not over 370° of the centigrade thermometer. 



The study of those partial alterations which. take 

 place in the vicinity of volcanic and older aqueous 

 masses of rock confirms these conclusions, so that we 

 may be said to know the precise conditions under 

 which sediments may be bardened into crystalline 

 rocks, while the bedded character and the alterna- 

 tions of different layers in the Laurentian rocks, as 

 well as the indications of contemporary marine life 

 which they contain, show that they actually are such 

 altered sediments. (See Note D.) 



