PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 139 



Aporphyolyte. Slate.] 



instead of being embraced by them. Its forms are secondary to the quartz. This is 

 true of the original quartz grains, yet there is a secondary quartz also, which 

 surrounds some of the biotite grains. This latter is that which spreads widely and 

 constitutes apparently the matrix. It contains bubbles. 



It is difficult to find a suitable name for this rock. Its relations to the gabbro 

 and to " red rock " are interesting and important, and will be found discussed in 



Part III. N. H. w. 



No. 44'. DIABASE. 



Duluth. Top of Kinichigaguag falls. Apparently the eastward extension of No. 43, and a part of No 41. 

 Ref. Annual Report, ix, page 17. 



Meg. This rock consists of plagioclase in lath-shaped forms, very fine, and of 

 a changed pyroxene, with magnetite and chlorite. 

 One section. 



Age. Cabotian. N. H. w. 



No. 45. APORHYOLYTE. 



Duluth. From a ravine between No. 44 and No. IB which is further north on the hill. 

 Ref. Annual Report, ix, page 17. 



Meg. An aphanitic, reddish brown, compact rock, holding small, red, porphy- 

 ritic feldspars. There are also scattered through the rock, small areas, sometimes 

 showing crystalline outlines, of a black mineral. 



Mic. In all essential characters this section is like No. 42. However, epidote 

 does not seem to be present in No. 45, and the porphyritic feldspars are in part not 

 so highly altered. None of these feldspars show polysynthetic twinning, although 

 many of them are so highly altered that no trace of this twinning would remain 

 even if it was originally present. The least altered of these porphyritic feldspars 

 show untwinned and simply twinned crystals. The alteration of these crystals is 

 usually toward chlorite, and some of them are entirely replaced by this secondary 

 mineral. Where this replacement has been entire the crystal appears black in the 

 hand specimen. 



One section. 



Age. Cabotian. 



Remark. This rock is similar to Nos. 42 and 850. u. s. G. 



No. 45'. SLATE (?) (Altered.) 



Duluth. Foot of Chester Creek falls. 

 Ref. Annual Report, ix, page 17. 







Meg. An aphanitic, dark, almost black rock, streaked with gray and reddish. 



Mic. The section is a confused mass of quartz, magnetite, chlorite, epidote and 

 rtt/i'ife. The quartz, which is in minute interlocking grains, forms the background of 

 section. Magnetite occurs in irregular grains and in minute dust-like particles. 



