404 THE GEOLOGY OP MINNESOTA. 



[Aporhyolyte . 



Meg. The rock is dark, brownish-gray in color, is aphanitic, and contains a few 

 small porphyritic quartzes and feldspars. It appears macroscopically like a dark 

 quartz-porphyry or aporhyolyte. The peculiar feature of this rock is that it contains 

 many pieces of a nearly similar but more quartzose and flesh-colored or reddish rock. 

 These pieces are firmly welded to the main rock and in some places no very sharp 

 line can be drawn between the two rocks as they blend together. The included 

 pieces are quartz-porphyry or aporhyolyte. 



Mic. The section is a small one and is assumed to have been made from the 

 main mass of the rock, although it may have been taken from one of the included 

 masses. The section shows a groundmass composed of the usual de vitrified material 

 of the aporhyolytes of semi-opaque, isotropic f eldspathic grains and areas of poikilitic 

 quartz. In a thinner section the feldspars are microlitic and frequently inspherulitic 

 arrangement. The other half of the same slide has the feldspars in a diabasic or 

 radiated arrangement, and lacks the quartzes. There are a few small quartz 

 phenocrysts, not distinctly idiomorphic, and some confused areas composed of cloudy 

 material, quartz, chlorite and iron ores, which seem to represent completely altered 

 feldspars. Several of the quartz crystals are separated into several parts, similar to 

 the corroded quartzes of many quartz-porphyries. The section shows small dark 

 rod-like bodies which are composed of chlorite and granules of iron ore. Hematite 

 and magnetite in small granules and crystals are abundant throughout the rock, and 

 there are also flakes of chlorite and a few flakes of biotifc. 



One section. 



Age. From the Beaver Bay diabase. u. s. G. 



Remark. This rock has complicated and interesting relations. That it is more 

 recent than the great amorthosyte and than the red rock pieces that it carries, goes 

 without saying, but it is not to be inferred that they were separated by important 

 geologic events. They were, as supposed, involved in the same grand eruptive 

 epoch. The abundance of genuine quartz-porphyry pieces shows the near proximity 

 of the great Cabotian quartz-porphyry formation, and it is perhaps most reasonable 

 to refer this rock to the fusion and incorporation in a basic flow, of Cabotian acid 

 eruptives. Thus its abnormal features maybe due to almost cotemporary eruptions 

 of differing degrees of basicity. A casual glance might lead an observer to take it 

 for a brown diabase. It is a phase of the Beaver Bay diabase, due to the inclusion 

 but not complete digestion of numerous fine pieces of the red rock series. The great 

 diabase of the region thus seems to have played a multiple role, as it acted now on 

 anorthosyte, now on Cabotian quartz-porphyry, now on Cabotian diabases and amy- 

 daloids* and now on clastic strata, all of which seem to have pre-existed in the 



* MR. EI.FTM AX is authority for the statement that in the midst of the great diabase sheet of the Beaver Bay region are 

 detached pieces of fine diabase and of amygdaloid. These are doubtless from the old Cabotian surface flows. 



