168 



MINERALS AND GEOLOGY 



tending over wide areas, are assumed, on the other hand, to have 

 taken place in many localities, without the direct intervention of 

 eruptive rocks. This widely extended metamorphism has probably 

 been effected by alkaline and other solutions acting on the heated 

 rocks, or by the agency of superheated steam and other vapours on 

 deeply-seated strata, or by other causes more or less immediately 

 connected with the presence of subterranean heat. In many cases 

 there can be no question as to these crystalline strata being really 

 altered sedimentary deposits, and thus, by inference, a similar origin 

 is generally attributed to all rucks of this character. Whilst sedi- 

 mentary rocks, proper, are the products of surface action, and erup- 

 tive rocks as regards their present condition, if not in all cases their 

 actual origin are products of internal or subterranean forces, meta- 

 morphic formations may be regarded as the result of both external 

 and internal agencies. 



The metamorphic rocks of Canada belong, as regards their geo- 

 logical position, to two essentially distinct series. The older series 

 of Archaean age, comprises the rock formations of the Laurentian 

 and Huronian periods, and occupies all the more northern and north- 

 western portions of Quebec and Ontario, its strata consisting chiefly 

 of enormous beds of gneiss, crystalline limestone, siliceous slates, and 

 other rocks, enumerated below, and described more fully in Part Y. 

 The higher or less ancient series is apparently intermediate in posi- 

 tion between the Huronian and Cambrian formations. Its strata 

 are chiefly developed in the form of chloritic and talcose slates and 

 beds of serpentine, throughout the Eastern townships and adjoining 

 region south of the St. Lawrence, in the Province of Quebec. 



The following are the more important metamorphic rocks of Cana- 

 dian occurrence : 



Gneiss : This rock is made up normally of three minerals quartz 

 feldspar and mica ; the two latter being generally the common potash 

 species, orthoclase and muscovite (See Part I.). In some districts, 

 however, the rock consists almost entirely of quartz and feldspar, 

 mica being absent or very sparingly present. In coarsely crystalline 

 varieties of the rock, the component minerals are easily recognized. 

 The feldspar is usually white or red, and is present in distinctly 

 cleavable grains or masses ; the mica is in leafy masses or small 

 scales of a silvery white, brown or black colour; and the quartz in 



