156 THE GEOLOGY OP MINNESOTA. 



[Apohsicliaii. 



The porphyrttic feldspars are highly altered and reddened and cannot be deter- 

 mined specifically. Frequently the quartz of the groundmass has penetrated these 

 phenocrysts, almost completely replacing them. In such cases the outline of the 

 original grain is shown by the increased deposits of hematite. From the analysis of 

 the rock as a whole, given below, it would seem that these phenocrysts were anor- 

 thoclase, or more probably a plagioclase rich in soda. 



In sections cut across the laminre the (jtnniz rc/itM* are very prominent. Most 

 of them are not continuous for great distances, but rapidly decrease in size and dis- 

 appear. They are recognized easily in ordinary light simply because in them the 

 opaque (feldspathic ?) and iron ore areas are lacking. In the smaller ones the quartz 

 is oriented similar to adjacent areas in the groundmass. But in the larger veinlets 

 the quartz is rather coarsely crystallized, one grain extending across the veinlet, and 

 is usually of orientation independent from the quartz of the surrounding ground- 

 mass. In the larger veinlets there is sometimes on each side a narrow layer of clear 

 quartz, then a layer of the feldspathic (?) material, and then the clear centre of the 

 veinlet. 



Aside from the appearance of flow structure given to the rock by the veinlets of 

 quartz, there are indications of flowage in other places, especially around some of 

 the porphyritic feldspars, but this flowage is not pronounced. 



Three sections. 



Chemical Analysis. The following analysis was made by Prof. J. A. Dodge, and was first published in 

 the Thirteenth Annual Report, page 100 (No. 152): 



SiO 2 73.72 



A1 2 O 3 12.82 



Fe 2 O 3 2.51 



FeO .22 



CaO 1.70 



MgO .35 



K 2 O 2.40 



Na 2 O 2.70 



H 2 O - .94 



Total - 97.36 



Age. Cabotian. 



'Remarks. This rock is referred without hesitation to a devitrified acid lava. 

 It is very similar to the acid lavas, some of which are yet glassy, found farther east 

 on the north shore of lake Superior at the Great Palisades and at Beaver Bay. Nos. 

 140, 138, 139, 127 and 129 represent these lavas from these localities, and these rocks 

 will be discussed more fully under these numbers. 



The origin of . the quartz veinlets is uncertain. They may simply represent 

 cracks in the rock or they may now take the place of old chains of spherulites. 

 These veinlets much resemble such bands of spherulites from the South Mountain 

 area in Maryland and Pennsylvania, and the slides were sent to Dr. Florence Bascom, 



