186 MINERALS AND GEOLOGY 



or Aphanite ; (4) Diorite or Granitoid Greenstone ; (5) Diabase or 

 Chloritic Trap. These varieties, it must be understood, merge more 

 or less into each other, so that in many instances a rock might be 

 referred with equal justice to two or more of their included types. 



Trap or Basalt may be denned conventionally as a black, greenish- 

 black or dark-grey rock of compact texture, composed of an intimately 

 blended mixture of lime-feldspar (or lime and soda feldspars), augite, 

 magnetic and titaniferous iron ores, zeolitic silicates, and carbonates of 

 lime and iron. In some kinds, the feldspar is replaced by nepheline ; 

 and olivine, in visible grains of a green or greenish -brown colour, is 

 very generally present in basaltic rocks. These component minerals 

 are deduced by calculation, it will of course be understood, from the 

 separate ingredients, silica, magnesia, lime,&c., obtained in the analysis 

 of the rock. Altered or weathered varieties of trap are frequently of 

 a dull brick-red or brown colour, the change being caused by the 

 higher oxidation of the iron. Unaltered basalts are always more or 

 less strongly magnetic, easily fusible, and partially attacked by acids. 

 Sp. gr. 3.0 to 3.1, but occasionally somewhat less from partial altera- 

 tion of the rock. 



The more common varieties or sub- varieties comprise : (a) Massive 

 or amorphous trap ; (b) /Slaty trap ; (c) Columnar and sub-columnar 

 trap (Pig. 99), the variety to which the term basalt is more generally 

 applied; (d) Amygdaloidal trap (Fig. 98), containing oval or other 

 shaped cavities mostly filled with agates, zeolites, calc-spar, green 

 earth, &c., as explained on a preceding page; (e) Porphyritic trap, 

 containing imbedded crystals of albite, augite, or other minerals. 

 Examples of massive and porphyritic trap (consisting in places of 

 almost pure augite or pyroxene, and hence termed " pyroxenite," by 

 some observers) occur more especially in the Montreal Mountain, 

 and in various parts of the Eastern Townships and lower St. Lawrence 

 district. Columnar and sub-columnar trap is abundant around 

 Thunder Bay ; and amygdaloidal trap is of common occurrence along 

 the northern and other shores of Lake Superior, the north shore of 

 Lake Huron, and elsewhere. The agates of Michipicoten Island, 

 Agate Isliiid, and- ether spots on Lake Superior, are derived from 

 the disintegration of these amygdaloidal rocks. 



Dolerite is simply a trap or basalt of granitoid structure, in the 

 coarse-grained varieties of which the component minerals are more or 



