42 



DIFFERENCE IN TEXTURE OF 



[Ch. Vi IL 



fossil contents and position, to be members of the Cretace 



ous 



and even of the nnmmnlitic or Eocene period. These strata 



ce 



had, in fact, acquired the transition texture from the infhien 

 of causes which, since their deposition, had modified their 

 internal arrangement. 



Texture and origin of Plutonic and metamorphic rocks. 

 Among the most singular of the changes superinduced on 

 rocks, we have occasionally to include the slaty texture, the 

 divisional planes of which sometimes intersect the true planes 

 of stratification, and even pass directly through imbedded 

 fossils. If, then, the crystalline, the slaty, and other modes 

 of arrangement, once deemed characteristic of certain periods 

 in the history of the earth, have in reality been assumed by 

 fossiliferous rocks of different ages and at different times, we 

 are prepared to enquire whether the same may not be true of 

 the most highly crystalline state, such as that of gneiss, mica- 

 schist, and statuary marble. That the peculiar character- 

 istics of such rocks are really due to a variety of modifying 

 causes is now very generally admitted, and the differences of 

 opinion among geologists which still prevail, relate chiefly to 

 the manner in which the transformation has been brought 

 about. According to the original Neptunian theory, all the 

 crystalline formations were precipitated from a universal 

 menstruum or chaotic fluid antecedently to the creation of 

 animals and plants, the unstratified granite having been first 

 thrown down so as to serve as a floor or foundation on which 

 gneiss and other stratified rocks might repose. Afterwards, 

 when the igneous origin of granite was no longer disputed, 

 many conceived that a thermal ocean enveloped the globe, at 

 a time when the first-formed crust of granite was cooling, 

 but when it still retained much of its heat. The hot waters 

 of this ocean held in solution the ingredients of gneiss, mica- 

 schist, hornblende-schist, clay-slate, and marble, rocks 

 which were precipitated, one after the other, in a crystalline 

 form. ~No fossils could be inclosed in them, the high tempe- 

 rature of the fluid and the quantity of mineral matter which 

 it held in solution, rendering it unfit for the support of 

 organic beings. 



It would be inconsistent with the plan of this work to 





b 



