648 



FRANK F. GROUT 



Conchoidally fracturing diabase. — This fracture appears only 

 in the base of most flows, but throughout the whole mass in a few 

 flows. The augite in these is less prominent and in small grains. 

 The feldspar, also fine, is labradorite, oligoclase, or even orthoclase 

 when recognizable. Olivine has not been recorded, but A. C. Lane 

 concludes from the form of some secondary minerals, that they replace 

 olivine. The composition indicates that olivine might develop, but, 

 as in the previous types, it is best to assume its presence in some, but 

 not all, of the rocks of this field type. 



TABLE III 

 Analyses of Conchoidally Fracturing Diabases 



1. Type locality, Bed 65, Eagle River section, Keweenaw Point, Mich. George 

 Steiger, analyst. Journal of Geology, XVI, 765. 



2. Crooked Creek, Pine Co., Minn. Average material. F. F. Grout, analyst. 



3. Tamarack Creek, Pine Co., Minn. Altered, but hard. F. F. Grout, analyst 



4. Mouth of Kettle River. Altered to red clay. Partial analysis by F. F. Grout. 



* CuO=0.02. 



Alteration giving hematite is probably responsible for the red 

 color of many outcrops. Sections reveal the fine-grained pseudo- 

 morphs after plagioclase, in a ground-mass, usually darkened by 

 hematite, especially where it seems probable an augite area existed. 



