268 THE GEOLOGY OP MINNESOTA. 



[Gabbro. Aporhyolyte. 



Mic. The feldspar in the sections examined is very much impregnated by quartz 

 and clouded by other alterations, so much so that it is sometimes impossible to state 

 its specific relations. Some of it is certainly triclinic, having yet remnants of the 

 characteristic twinning of the albite type. There are some large plages, however, 

 having more nearly a square outline that are more abundantly supplied with the 

 pegmatitic quartz, which may- be orthochtsc. Such acid feldspar, however, cannot be 

 considered an original integral portion of the rock, but only as accessory, and due to 

 the proximity of contact on the elastics. This fact, i. e., the contact with the sedi- 

 mentaries, manifests itself in many places in a silicification of the other (original) 

 elements of the basic eruptives and in the appearance of orthoclase. 



The any He seems to have begun its formation from the magma cotemporane- 

 ously with the feldspar, its form in some cases being independent of the feldspar, 

 and even being embraced in a certain manner by the feldspars whose outlines give 

 way before it. Yet, in other places, there seems to have been a later generation of 

 augite, which now is seen to fit itself into the interstices between the feldspars. In 

 all cases it is much altered. 



The quartz is so arranged in the feldspars as to form a microscopic pegmatitic 

 structure, numerous small areas extinguishing at the same moment, each small sec- 

 tion sometimes being triangular and aligned with others as if their location had been 

 determined by the cleavage of the feldspar, although, at present, there is no remain- 

 ing trace of such cleavage. Quartz also appears otherwise as independent, sizable 

 grains. 



Magnetite in the usual form and quantity is distributed throughout the section. 



Two sections. 



Age. Cabotian. 



Remark. This is put in the Cabotian because of its obvious association with 

 Nos. 221, 222 and 223, both petrographically and structurally. It is presumed to be 

 a contact phase of the same. This is in keeping with the assignment of No. 5 at 

 Duluth, which is put in the Cabotian because, starting with the gabbro at Duluth 

 as Cabotian, its contact apophyses would also be Cabotian, especially as there we find 

 the upheaved Animikie and red rock associated with it. Still, there would be noth- 







ing unreasonable in expecting Manitou contact phenomena similar, if not identical, 

 with those of the Cabotian. N. H. w. 



No. 226. APORHYOLYTE (?) 



From an isolated dike-like exposure on the beach in the next shallow bay east of Double bay; N. W. % 

 S. W. % sec. 12, T. 62-4 E. This rock is closely jointed and basaltic. 

 Ref. Annual Report, ix, pages 57, 58, 59. 



A fine-grained, reddish-brown rock, which is made up of small, reddish 

 feldspars and a dark substance. It is porphyritic with small, reddish feldspars, and 



