166 



MINERALS AND GEOLOGY 



Coal has been thus converted, within a certain distance of interpene- 

 trating trap masses, into cinder and coke; earthy limestone, into- 

 crystalline marble ; sandstone into quartz rock, and so forth ; and 

 somewhat analogous effects are occasionally produced in sandstone- 

 blocks that have been long exposed to heat and heated vapours in the 

 interior of certain furnaces. These effects, however, have not always 

 followed the intrusion of eruptive rocks; and in no case do they appear 

 to have extended far into the mass of the surrounding strata. The 

 alteration of extensive regions therefore, such as the wide area occu- 

 pied by the Laurentian strata of Canada (see Part V.), points evidently- 

 to some more general although probably related cause, in explanation 

 of the facts of metamorphism.* Whatever view be adopted respecting 

 the internal condition of the earth, it is clear that immense spaces 

 filled or partially filled with molten and vaporous matter must have- 

 existed through untold ages at certain depths beneath the surface 

 rocks ; and the chemical action going on within these spaces, and 

 emanating from them, may be regarded as sufficient to produce the- 

 results in question, even if we cannot explain, to our thorough satis- 

 faction, the actual processes involved in the production of these effects. 

 See further under the METAMORPHIC ROCKS described below. 



A special effect of metainorphism, developed moie particularly in? 

 fine-grained argillaceous strata, is the production of slaty cleavage* 

 Rocks thus affected, exhibit a more or less strongly-pronounced fissile 

 texture, arising from the presence of numerous divisional planes run- 

 ning parallel with one another through the rock, and usually in a 

 direction inclined to that of the planes of deposition. It is not 

 always easy, in inclined strata, to distinguish the latter planes from 

 planes of cleavage; but their direction is generally revealed by the 

 presence of fossils, or by intercalated layers of a different shade of 

 colour, degree of fineness, &c., across which the cleavage lines com- 

 monly pass without interruption. Cleavage in rocks, as shown by 

 this latter condition, and by the fact that fossil bodies and imbedded 

 stones are frequently drawn out or unnaturally elongated in th& 

 direction of the cleavage planes, is evidently a superinduced effect ; 

 but much obscurity still prevails with regard to its actual origin. 



* We follow here the generally received view respecting the nature of these stratified, or at 

 least laminated, crystalline rocks. But the assumed metamorphic character of these rocks is 

 contested by some observers in Canada, notabiy by Mr. Thomas Macfarlane who regard 

 them as original formations. Impartially considered, this view is not without strong grounds 

 of probability. 



