30 



thoroughly crystalline, obliterating at the same time any organic 

 remains which they may have, and in many cases probably did contain. 

 In any great region of crystalline rocks, although now tolerably constant 

 in lithological chai-acters, there may be, according to this theory, altera- 

 tion products of rock, of various ages ; and the same great series may 

 in one part of its distribution be represented by uncrystalline fossilifer- 

 ou"? sediments, and in another by thoroughly crystalline rocks, such as 

 gneiss or hornblende schist. Two ways have been suggested in which 

 this alteration might take place. 1st. The alteration has, according to 

 Zell, been induced principally by the internal heat of the earth acting on 

 deeply buried sediments, or, 2nd. According to Bischoff, the alteration 

 may have been due, not primarily to any recrystallization by heat, 

 though this may also have helped to effect the change, but to tbe action 

 of percolating waters from higher levels holding salts in solution, and 

 acting either at ordinary temperatures or at temperatures higher than 

 we ordinai-ily experience at the surface of the earth. According to 

 this theory, we might have the same bed of limestone, according to the 

 nature of the circulating mineral solutions, in one place altered into 

 pyroxenic or amphibotic rock, in another into a garnet or epidote rock, 

 and in a third into a quartz-feldspar rock.* It is, of course, supposed 

 that the action has been continued through immense periods ot time, 

 but, although the time which has elapsed since the Silurian is enormous, 

 it is not to be taken into account in this connection, since in the case 

 of OUT Laurentian, at any rate, the metamorphism m\ist have been com- 

 pleted before the IFuronian rocks were deposited, since, the latter in 

 many places hold rounded pebbles of the former, undistinguishable from 

 the rock as elsewhere found in place. Those who now hold this theory 

 generally consider that both the interior heat of the earth and the 

 action of percolating waters have played their part in the alteration. 

 Some remarkable experiments on the effects produced in solid bodies by 

 the action of great pressure, lately made by Spring,t throws much light 

 on a possible method of the crystallization of sedimentary rocks. 

 Among numerous other interesting results, he found that peat, when 

 submitted to a pressure of 6,000 atmospheres was converted into a hard 



* See Credner. Elemente der Geologic p. 213. 



t Bull, de I'Acad. Royal des Sci.,etc., de Belgique, 1880, 2nd Ser. vol. XLIX. 



