1867.] MACFARLANE — GEOLOGY OF LAKE SUPERIOR. 185 



which encloses or penetrates older ones, the more siliceous it 

 becomes. On reference to the specific gravities above given of 

 the various rocks, it might be supposed that their relations as to 

 age might be equally well expressed by saying, the older the rock 

 the heavier ; the more recent, the lighter it is ; and, in the 

 majority of instances, this applies. But, as in the case of the rock- 

 aggregate occurring to the west of Michipicoten Harbour, when 

 we come to the very newest granitic veins, consisting only of ortho- 

 clase and quartz, those are the heaviest which contain most of the 

 latter mineral, its mean specfic gravity being 2-65, while that of 

 orthoclase is only 2-55. It is to be remembered that these newest 

 veins are altogether different in appearance from certain veins of 

 large-grained granite, with distinct side joints, which are occasion- 

 ally found intersecting these rocks, and the origin of which has 

 been indicated by Dr. Hunt in his recent valuable report on 

 mineral veins. Near Point-aux-Mines a vein of this nature is 

 found, the rock of which is pegmatite, consisting of orthoclase, 

 quartz, and greenish white mica, together with occasional grains 

 of purple copper, copper pyrites, galena, and molybdenite. 



It may not be out of place here to advance certain considera- 

 tions regarding these Laurentian rocks, and especially concerning 

 the peculiar rock aggregates just described. The relations of 

 these rocks to each other we have seen to be .as follows : — The 

 older the rock the more basic is its nature, and the richer it be- 

 comes in triclinic felspar, hornblende, and mica. The newer the 

 rock the more siliceous it becomes, and the more such minerals as 

 orthoclase and quartz predominate. It can scarcely be supposed 

 that this relation is an accidental one, for it is observable in every 

 one of the instances above given, the localities of many of which 

 arc very far distant from each other. It would seem to be the 

 consequence of an unvarying law which was in operation at the 

 time when these rocks were first formed. At first sight, the facts 

 above described would appear to militate against the idea of the 

 igneous origin of these rocks, and, in fact, the relation is a similar 

 one to that which has been observed among the constituent 

 minerals of granite, and which is one of the chief difficulties in 

 explaining the origin of that^ rock on the igneous hypothesis. In 

 granite the quartz is frequently found filling up the interstices 

 between the other minerals, and sometimes it even retains impres- 

 sions of the shape of the latter. Nevertheless the felspar and 

 mica are the most fusible, and the quartz the most infusible of 



