GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



327 



spindles constitute a considerable portion of, and a very charac- 

 teristic feature of the rock. Though no analyses have been made, 

 the rock is plainly an exceedingly acid one 



The fine grained phase is however the more common one and is 

 not so easy of recognition. It has the same mineral constitution 

 as the coarse, but largely or entirely loses the spindle quartz 

 character. Intermediate grades however occur. In small dikes 

 it becomes a very finely granular, flinty appearing rock, and. 

 where such dikes occur isolated, there may be considerable ques- 

 tion as to their proper reference. 



The fact that the two are merely phases of the same rock is 

 shown at several localities, typically perhaps on the west shore of 

 Big Tupper lake between Grindstone and Black bays. Here are 

 -excellent exposures which show the typical coarse granite cutting 

 augite syenite, with the fine grained type produced as a contact 

 phase, and constituting only a small proportion of the w^hole mass 

 of the granite. Other exposures show the two in varying pro- 

 portion, though as a whole the coarse type is less abundant than 

 the fine. 



This Morris granite is the only granitic rock among the later 

 intrusives which belongs to this very acid type, and this makes it 

 easy of recognition when it is associated with rocks belonging to 

 this group, since it is the youngest of them all, with the possible 

 exception of some of the gabbros. But gneissoid granites are not 

 infrequently found in the region which cut Grenville or Saranac 

 rocks and with none of the distinctive later intrusives in the 

 vicinity, granites of a very acid type. When these are of the fine 

 grained sort, as is usual, it is impossible to tell whether they are 

 of the age of the Morris granite or are much older, older than any 

 of the later intrusives. Such granites are quite frequent in the 

 region, and have perhaps a specially wide distribution in the 

 vicinity of St Regis Falls. It is quite probable that there is more 

 than one granite of this character in the region. 



Gabbros. These arc mostly black, basic, heavy rocks, and have 

 a very widespread distribution, perhaps more so than any of the 

 later igneous rocks, but occur mostly in dikes or small masses, 

 very seldom in masses of such size as are common with the other 

 intrusives. The dikes are without exception fine grained black 

 rocks. The central parts of the bosses are much coarser, the 



