PETROGRAPHY OF THE KEWEENAWAN 



647 



of alteration of laumontite. Where excellent crystals of laumontite 

 are found on Snake River, some perfect pseudomorphs were found 

 with an entire change in certain constituents. Lime is lacking in the 

 secondary mineral and potassium is prominent. In several parts of 

 the area amygdules were found of similar character to this pseudo- 

 morph, and an analysis shows the chemical constituents to be similar. 

 This development of a potassium mineral may not be a conclusive 

 proof of the former presence of laumontite, but adds probability to 

 the idea. In other places and other rocks, the potassium minerals 

 found have very different characters. Further, on Upper Tamarack 

 Creek this same mineral is largely developed with chlorite in a pseudo- 

 amygdaloid, which may be a late derivative of the laumontite pseudo- 

 amygdaloid. Analyses of these minerals are given in Table VI, but 

 of the rocks in Table II. 



TABLE II 

 Analyses of Hackly Fracturing Diabases 



1. Taylors Falls, Minn. Rather fine grained. C. A. Taylor, student analyst. 



2. Crooked Creek, Pine Co., Minn. C. Tronson, student analyst. 



3. Pine City, Minn. Coarse and fresh. F. F. Grout, analyst. 



4. ) ^ Altered hackly diabase, forming a belt of laumontite pseudo-amygda- 



r ■\ loidal outcrops, across the area of Keweenawan in Minnesota. 

 ) ( Grout, analyst. 



F. F. 



* CuO = o.03. 



