24 THE CANADIAN NATURALIST. [Feb. 



unaltered, rather than among metamorphic strata. It is of course 

 possible that a crystallization of the sediments may in some cases 

 take place subsequent to the eruption of foreign rocks into their 

 midst. The rarity of intrusive rocks among crystalline strata, 

 rQot less than the unaltered condition of sediments which are tra- 

 -versed by abundant intrusive masses, is a strong proof of the fal- 

 lacy of the still generally received notion which connects meta- 

 .morphism with the contiguity of eruptive rocks. 



II. Classification and Nomenclature. 



it is proposed in this second part, to describe briefly the com- 

 position, structure, and nomenclature of the various crystalline 

 silicated rocks, considered without reference to the distinction be- 

 tween indigenous and intrusive masses. Comparatively few of 

 these rocks are homogeneous, or consist of a single mineral species, 

 and the names which have been applied to varying mixtures of 

 different species are of course arbitrary ; and as they have 

 often been given without any previous mineralogical study, it some- 

 times happens, that, as in the case of the rocks composed of 

 anorthic feldspars and pyroxene, different names have been pro- 

 posed for varieties very closely related, or differing from one 

 another only in texture or in structure. 



The minerals essential to the composition of the rocks under 

 consideration are few in number, and are as follows : quartz, or- 

 thoclase ; a triclinic feldspar which may be albite, oligoclase, 

 andesine, labradorite, or anorthite ; scapolite, leucite, nepheline, 

 sodalite ; natrolite, or some allied zeolite ; iolite, garnet, epidote, 

 woUastonite, hornblende, pyroxene, olivine, chloritoid, serpentine, 

 diallage ; muscovite, phlogopite, and some other micas ; chlorite, 

 and talc. To these may be added as accidental ingredients, the car- 

 bonates of lime, magnesia, and protoxyd of iron, together with 

 magnetite, ilmenite, and sphene. The silicates which, like tourma- 

 line, beryl, zircon, spDdumene, and lepidolite, contain considerable 

 portions of the rarer elements, and often occur with quartz and 

 feldspar in granitic veins, whose origin has already been alluded 

 to, enter at most in very small quantity into great rock-masses. 



The varieties of structure in crystalline rocks are the more 

 deserving of notice as they have led to a great multiplication of 

 names. We may note first the granitoid structure, in which the 

 mineral elements are distinctly crystalline, as in granite. From 



