232 THE GEOLOGY OF MINNESOTA. 



[Amygdaloid. Diabase. 



defined by Michel Levy. But where it has not grown up in sufficient amount to 

 replace the olivine crystal entirely, the other minerals, such as bowlingite and thalite, 

 have simply been stained by it about their peripheral portions. 



Throughout the decayed rock in which these two forms of bowlingite occur (i.e., 

 the crystalline cleaved variety and the fibrous), is an abundant dissemination of the 

 mineral frequently known as saponite. This is a light-green, massive or finely 

 fibrous, soft and soapy substance, whose fibres have a positive elongation, and which 

 cannot be distinguished easily from the light-colored or greenish central areas of the 

 changed olivines. It is the same substance (in part at least) that Dr. D. D. Owen 

 named thalite (compare No. 91B). Between crossed nicols it is frequently nearly 

 isotropic, a character which is due to its finely comminuted or massive structure, but 

 in numerous instances also it is radiatedly and very finely fibrous. Careful examina- 

 tion has shown that its elongation in this condition is positive. There seems to be 

 sufficient warrant for identifying it with the similar mineral which forms the central 

 isotropic areas in the changed olivines, in the midst of which occasionally a positive 

 fibrous elongation is apparent. 



Although the crystalline, the fibrous and the massive conditions, whether 

 greenish or brownish, seem to belong to the same mineral, only differing in the 

 amount of iron oxide present as a coloring element, yet for the present it will be best 

 to employ two terms, bowlingite and saponite [thalite], the former for the cleavable 

 crystalline condition (whether iron stained or not) and the latter for the fibrous or 

 massive, usually greenish, condition. 



Wadsworth's reference of the brownish mineral in No. 144 to hisingerite, found 

 in the basalt of Ovifak,* could hardly apply to the cleavable mineral here referred 

 to bowlingite, since hisingerite is non-cleavable, black, or nearly black, and has a 

 conchoidal fracture. 



The mineral lately described by Lawson (iddingsite) is placed doubtfully, by 

 Lacroix, under bowlingite, but he states that there are such divergences between 

 them, as far as now known, that they cannot with certainty be identified. These 

 consist chiefly in the greater specific gravity of iddingsite, which is given by Lawson 

 at 2.839, whereas that of bowlingite is 2.300. See further respecting this mineral 

 (bowlingite) No. 193. 







Age. Manitou. N. H. w. 



No. 168. DIABASE. ( AnujgdaloidaL ) 



From Sugar Loaf point, S. E. i| sec. 21, T. 58-5. 

 Ref. Annual Report, ix, pages 42, 43, 44. 



Meg. This number embraces two rocks of slightly different aspect, though 

 closely associated in structural relations and origin. They both constitute parts of 



* Bulletin ii (Minnesota Survey), p. 99. 



