236 THE GEOLOGY OP MINNESOTA. 



[Diabase. Amygdaloid. 



is also a mass, apparently filling a crack, of a mineral similar to one described under 

 No. 163A. Cleavage flakes of this show a positive, biaxial interference figure, as 

 does heulandite, but this mineral does not seem to have as perfect a cleavage as does 

 heulandite. Some of this shale is made up of angular bits of similar shale embraced 

 in a matrix which itself is coarser grained. No section. 



Age. Potsdam. u. s. G. AND N. H. w. 



No. 168. DIABASE. (Amygdaloidal.} 



Temperance river. Same as No. 169, but taken higher in the beds. 

 Ref. Annual Report, ix, pages 45, 46. 



Meg. A fine-grained, brown rock with numerous rather small amygdules. The 

 amygdules are filled with tlialitc, en/cite and Jicii/nm/ifc. The first mineral is more 

 abundant than the others, and it has also penetrated the mass of the rock. No section. 



Afje. Manitou. u. s. G. 



No. 169. AMYGDALOID. 



From Temperance river, about a mile above its mouth. Upper surface of an amygdaloidal layer, rising 

 like a dome near the water, and exposing three feet. 



Ref. Annual Report, ix, pages 45, 46; Bulletin ii, page 117. 



Meg. An amygdaloid similar to No. 168. The calcite amygdules are surrounded 

 by thalite, which also permeates the rock. No section. 



Age. Manitou. N. H. w. 



No. 170. DIABASE. (Surface of lava flaw, wit It z/rl>vl>/te.J 



Near the mouth of Temperance river. 



Ref. Annual Report, ix, page 45. This rock was recorded as a museum specimen, with the No. 3581, and 

 was so published. 



Mey. The outward characters of this rock are best described by the following 

 from the field description: 



There is also a marking on the upper surfaces of some of the amygdaloidal beds 

 which seems to show the effect of cooling from a molten condition. These marks or 

 wrinkles are transverse to the direction of the dip. They are in a fine-grained rock, 

 though on the upper surface of the amygdaloid layers, and seem to be of the same 

 kind of rock, though redder, as the amygdaloid itself. They are seen at four 

 different horizons, and overlie uniformly beds of a foot and a half up to three feet 

 and a half of amygdaloidal trap, with which they are connected by slow changes into 

 the same structure. They are themselves somewhat amygdaloidal, but with much 

 finer and fewer amygdules. There is sometimes a thin belt or interrupted stratum 

 of highly and coarsely vesicular and amygdaloidal rock immediately under the 

 wrinkles, which causes the separation of sheets of the wrinkled finer rock from the rest 

 of the bed. These wrinkled surfaces, which are transverse to the supposed flow of 

 the molten rock toward the Lake Superior basin, may have been caused by the 

 superficial cooling of a film of rock on the surface of the flowing lava. The lava 



