GEOLOGY OF THE LONG LAKE QUADRANGLE 465 



dorfte. They range from fine grained, heavy, resistant rocks, to 

 coarse, well foliated masses with conspicuous platy hornblende, 

 which are weakly resistant and easily decayed. In the former t3^pe 

 there is apt to be considerable pyroxene in addition to the horn- 

 blende. In the latter black mica is pretty sure to develop, some- 

 times in considerable quantity, assisting the plat}^ hornblende in 

 the development of well marked foliation. 



In many cases rocks, distinctly intermediate in character be- 

 tween these amphiboHtes and the granitic gneisses, have been 

 observed. In no case have they been seen to acquire large bulk 

 and in no case has it been possible to definitely determine their 

 relationships. But since the amphiboHtes seem at times to shade 

 into the granites through intermediate rocks of the sort, it is quite 

 likely that we are dealing with impregnation of one rock by the 

 other, with the effect disguised and equalized by the subsequent 

 metamorphism . 



In all cases where these amphiboHtes occur in considerable masses, 

 comparatively unmetamorphosed cores are found which show typi- 

 cal gabbro (hyperite) as the original rock. All such found have 

 been mapped as gabbro, both the unchanged core and the sur- 

 rounding amphibolite being included. In the case of the smaller 

 masses such definite evidence of origin is lacking, and all such have 

 been included in the general mass of the gneiss. Yet they seem 

 quite certainly to represent the same rock, in the one case only 

 partly, in the other wholly converted into amphibolite by meta- 

 morphism. A very accessible mass of such amphibolite is that 

 composing the island toward the lower end of Long lake on which 

 the Island House stands. It does not run into gabbro an3^where 

 within the limits of the island, though it may do so under the 

 waters of the lake near by. On all near-by points on the lake shore 

 the granitic gneisses appear. 



For the most part then the Long lake gneiss consists of two 

 sharply contrasted varieties of gneiss, both of which are unques- 

 tionably of igneous origin. There then arises the question as to 

 their age relations to each other, and to the other igneous rocks of 

 the quadrangle. 



It may be stated in the first place that small masses of similar 

 rocks are found involved with the Grenville sediments, and ap- 

 parently cutting them intrusively. So far as it goes this impHes 

 their later age, but in the uncertainty prevailing as to the equiva- 



