METAMORPHISM. 177 



latter, some form beds of more or less 

 extent in the midst of calcareous forma- 

 tions, others fill wide apertures of little 

 depth, from above, which sometimes 

 communicate with caverns (fig 287) ; Fi 2 87.- 

 but these tacts are oi a different order j- rom t ^ e exier i or , 

 from those just described. 



24. Metamorphism. From all the facts we have cited (which 

 might be vastly augmented in number by reference to details in 

 many localities), \ve must conclude that crystalline rocks, which 

 ire all formed of si'licates, extensively varied and mixed with each 

 )ther, have been produced by the action of fire ; that at different 

 epochs they have dislocated, uplifted, or overturned the sediment- 

 try deposits, modifying the mass in all manners and it is to these 

 great phenomena that are due all the seeming disorder observed 

 on the surface of the globe, as well as all the successive changes, 

 vhe traces of which may be perceived at every step. 



When we see earthy or compact limestones become crystalline on the 

 approach of these different kinds of rocks to fill with various substances 

 Ihcy do not contain at certain distances to be charged with magnesia in 

 Tracking in all parts, and to disintegrate with more or less facility ; when 

 schistose clays and arenaceous substances are converted into different 

 jaspers, and become charged with mica and am'phibole, and assume the 

 characters of gneiss, of mica'ceous or talcose schist ; finally, when sand- 

 stones are transformed into beds of solid quartz, can we be surprised that 

 most modern geologists have: adopted the idt a of complete changes effected 

 in a great number of sedimentary deposits, and that they resort to this 

 metamorphism^ long since perceived by Hutton, Playfair, and Dr. Macul- 

 loch, to explain a multitude of facts, observed especially in deposits anciently 

 designated under the names of primitive and transition formations ? The 

 facts appear so extraordinary, that we may be led to suppose a little ex- 

 ajrgeration : but we must reject evidence to deny that there are saccharo'id 

 limestones, dolonjites, mica-schists, gneiss, granular quartz, &,c., which are 

 the result of a change produced in earthy or compact limestones, clays, 

 sands, &c. of sedimentary formation : is it then so ridiculous to suppose 

 that such has been their origin in all cases? 



These ideas, now more striking, because they are expressed by a proper 

 word, are nevertheless not absolutely new; all works on geology are actually 

 full of them, and the facts are not less remarkable from being expressed in 

 other terms. There is no description of a country, going back to the time 

 of Saussure, whose works are still remarkable for their fidelity of details, 

 in which are not seen numerous passages of different arena'ceous deposits 

 to rocks of crystallization, of schistose grauwackes to talcose schists, to 

 mica'ceous schists, and from these to gneiss, or the passage of sandstone to 

 different kinds of granite and porphyries on which they rest, &-c. Is not 

 the *act of the modifications, now described under the term of metamor- 

 phism, here clearly indicated to which time has added only more details 

 and greater precision ? 



It is certain that in departing from schistose grauwackes, for example, 

 and going towards some mountain or islet of crystallization, we finu these 



24. What is meant by metamorphism ? Of what do crystalline TOCK* 

 consist ? 



