OF CENTRAL CANADA PART III. 169 



colourless vitreous grains. The striped or banded aspect of the rock 

 generally serves to distinguish it, in hand specimens, from granite 

 -and when seen in position, its stratified structure is in most cases 

 very apparent. Vast beds of gneiss, and strata of gneissoid rock in 

 which the component minerals are more or less indistinct, occur 

 throughout the wide area occupied by the Laurentian rocks of the 

 more northern regions of Canada (see Part V.), and also here and 

 there, in the less ancient crystalline district south of the St. Law- 

 rence. Most of the boulders scattered so abundantly over the sur- 

 face of Canada, consist of micaceous gneiss, or of the hornblendic 

 variety described below. In some localities the mica of ordinary 

 gneiss is partially replaced by scales of graphite. 



Syenitic or Hornblendic Gneiss : This rock only differs from 

 ordinary gneiss by containing hornblende in place of mica ; but the 

 two rocks frequently merge into one another, both hornblende and 

 mica being present in certain varieties. Normally, the hornblendic 

 variety of gneiss is composed of red or grey feldspar, with quartz, 

 and black or green hornblende. The three minerals are sometimes 

 very distinct ; but in other cases they are intimately blended, so as 

 to form a dark-green rock, which passes, by the gradual diminution 

 of the quartz, into hornblende-slate or amphibolite. Syenitic gneiss 

 occurs abundantly, with ordinary or micaceous gneiss, in the Lauren- 

 tian districts of Canada (see Part V.). 



Mica Slate : This is H foliated or schistose rock, composed essen- 

 tially of quartz and mica. It is generally of a dark-grey, greyish- 

 green, or silvery- white colour, and occasionally black ; and some 

 varieties are highly lustrous from the presence of intermixed graphite, 

 -as in many parts of the " Eastern Townships " of Quebec. It passes 

 into clay-slate, and also into fine-grained gneiss and other rocks of 

 this series. Mica-slate occurs here and there throughout the Lau- 

 rentian area of Canada, and in the crystalline districts south of the St. 

 Lawrence (see Part V.) ; but characteristic examples are rare the 

 rocks in question being rather micaceous slates than mica-slate as 

 commonly defined. 



Pyroxenite or Augite-Rock : This rock, of subordinate occurrence, 

 consists at times of almost pure augite or pyroxene, but in general it 

 forms a granular compound of augite and some kind of feldspar, more 

 or less intermixed with carbonate of lime. Frequently, also, it con- 



