THE GEOLOGY OP MINNESOTA. 



[Diabase. Granite. 



No. 132. DIABASE (uith olivine). 



Beaver Bay. Holds feldspar (anorthos) to) masses. 

 Kef. Annual Report, ix, pages 33, 34. 



Meg. An ordinary olivine diabase of medium grain. The rock needs no further 

 description. It is part of the great " trap " sheet of the Beaver Bay region which 

 here encloses masses of anorthosyte. 



One section. 



Age. Cabotian. Compare Nos. 131 and 532. r. s. G. 



No. 132A. GRANITE (dud />//> ;/o////r.- in<-/i/xi<i ). 



This red or light red rock is embraced in No. 132 in the manner of nodules and patches, ^and in veins 

 along the joints running in different directions across the face of the rock. 

 Ref. Annual Report, ix, page 33. 



Meg. The specimens at hand consist of coarsely radiated nodules, suggesting 

 stilbite, but they are not of a homogeneous mineral. Indeed, the radiated structure 

 breaks up into a granitoid structure which prevails wholly on one-half of the bulk 

 of the specimen, making a reddish granite. In the midst of the red coarsely-radiated 

 rock are porphyritic quartzes and magnetites, and spicules of a green mineral which 

 do not run always parallel with the structure, but cross it at various angles. Where 

 the radiated structure breaks up into the granular, the red substance of the mass is 

 seen to maintain a pegmatitic relation to quartz, which latter is in zigzag and 

 angular grains and strings. 



Mic. The most of the section presents a feldspathic reddened aspect, as if it 

 resulted from an orthoclasticrhyolyte. Quartz is abundant, and in the form of isolated 

 grains as well as a pegmatitic growth. In the former condition it has controlled the 

 orientation of the poikilitic quartz surrounding it. There are some quartzes that 

 have embayments and cavities that have been filled with a structure that appears to 

 be the same as the orthoclastic substance surrounding them. The appearance is that 

 of a sub-crystalline magma from which this quartz first consolidated. The reddened 

 substance in many cases is spherulitic, at least fibrous, and the quartz orientation 

 and extinction prevail over considerable areas of these fibres. In other places the 

 fibres break up into a more granular structure, the quartz areas become large and still 

 poikilitically spread over the surrounding orthoclastic substance, which, in other 

 places, assumes a parting, resembling an incipient cleavage and an orientation of its 

 own, though still clouded by the red impurities. The strongly radiated aspect of the 

 hand sample is seen to be due to a spherulitic growth of the orthoclastic ingredient 

 of the rock. It is an interesting fact that here the spherulitic form gradually 

 assumes a granular one, and that the secondary nature of the poikilitic quartz in 

 the latter is as evident as in the former. 



