OF CENTRAL CANADA PART III. 167 



It is usually attributed to mechanical pressure acting laterally upon 

 the rock during elevations or depressions of contiguous areas ; but it 

 may be really due to the effect of long continued heat on confined 

 masses of damp strata. Moist clay, for example, if exposed in a 

 covered vessel to a certain degree of furnace heat, almost invariably 

 assumes a fissile texture ; and the same peculiarity is observable in 

 ordinary biscuits, and more especially in those which have undergone 

 an extra firing for ships' use. Oblique cleavage is exhibited by 

 many of the clay-slates of the Eastern Townships, as those of Mel- 

 bourne, Cleveland, Kingsey, &c. : but the clay slates of Lake Superior 

 and other parts of the province, though more or lesa finely laminated, 

 appear to be entirely destitute of true cleavage planes of this 

 character. 



III. METAMORPHIC OR CRYSTALLINE STRATIFORM ROCKS. 



The rocks of this series are stratified or laminated rocks of a more 

 or less crystalline aspect. In their mineral characters they frequently 

 bear a great resemblance to eruptive rocks, to which indeed they are 

 closely allied almost every metamorphic rock having its repre- 

 sentative in the eruptive series; but they differ from these latter 

 by their general conditions of occurrence. As explained above, many 

 sedimentary strata are seen to have assumed a crystalline texture, or 

 to have lost more or less completely their normal sedimentary aspect, 

 in the vicinity of intrusive masses of granite, greenstone, or other 

 eruptive rocks. An alteration of this kind is known as local meta- 

 morphism. Earthy or ordinary limestones and dolomites are thus 

 occasionally converted into hard crystalline marble, often veined 

 with green and other coloured streaks and patches of serpentine, and 

 filled in many cases with crystals and crystalline particles of graphite, 

 pyroxene, amphibole, various micas, tourmaline, garnets, pyrites and 

 other minerals, foreign to the rock in its sedimentary condition. In 

 like manner, sandstones are changed in colour and texture, and are 

 often converted into quartz-rock or some variety of gneiss ; and clay- 

 slates are transformed into mica-slate, talc- slate, hornblende-rock, 

 and other so-called crystalline schists and gneissoid aggregations. 

 These metamorphic results are probably due in part to the agency 

 of various gases and heated vapours which accompanied the protu- 

 sioii of the eruptive mass. Alterations of a similar kind, but ex- 



