328 THE GEOLOGY OF MINNESOTA. 



[Actinolite schist. Serpentine. 



No. 348. ACTINOLITE SCHIST. 



Ridge and rapids at the mouth of Pipestone river, where the water comes down to the level of Basaimonan 

 lake, the descent being about ten feet. S. W. % sec. 22, T. 64-11 W. 



Ref. Annual Report, ix, page 90: Annual Report, x, pages 89, 95; Annual Report, xv, pages 104, 105. 



Meg. A tough greenish schist, crushing under the hammer like a chloritic 

 schist; irregular and broken by jointage planes in different directions, and confused 

 by a slight schistose structure, rather fine grained. 



'Mic. The slide is composed, almost entirely, of a network of spicules of green 

 hornblende, which are not so compactly interwoven but that in their meshes can be 

 seen grains of feldspar and of quartz. They do not have any prevalent direction of 

 elongation, but overlap and cross each other as if accidentally thrown together. 

 They show usually very faint dichroism, or none, but occasionally a marked dichroism 

 is observable; a little sphene is also seen. 



Two sections. 



Age. Archean (Lower Keewatin). 



Remark. This is a common rock in the Lower Keewatin, lying next above the 

 igneous portion of the Kawishiwin. It is probably derived from an ancient sedi- 

 ment in which volcanic debris and erosion products were mingled. It is the horizon 

 which contains elsewhere the most of the Keewatin jaspilyte. N. H. w. 



No. 349. SERPENTINE. 



Pipestone rapids, a short distance above the rapids, in the right bank which rises about two feet above 

 the river. S. W. % sec. 22, T. 64-11 W. 



Ref. Annual Report, ix, page 91; Annual Report, x, page 95; Annual Report, xv, pages 104, 105; Annual 

 Report, xvi, page 111; Bulletin ii, page 29. 



Following is Wadsworth's description of this rock (op. tit.): 



Meg. " The hand specimen is a compact, dark-green rock, traversed by veins of 

 talc and dolomite and coated in places by a limonitic deposit. 



Mic. " The section shows a pale grayish and yellowish green groundmass, 

 traversed by a reticulated network of magnetite, and cut by a dolomite vein. The 

 magnetite preserves in part the outlines and fissures of the original olivine grains, 

 while the groundmass itself is composed principally of a pale greenish isotropic 

 serpentine, talc scales and fibres and magnetite granules. The talc is in single plates 

 and in aggregations of fibres. The general character of the rock is similar to the 

 serpentines of Michigan, New Jersey and Massachusetts." 



Having had another section prepared we are able to add to the foregoing 

 description. The term "serpentine" here may be employed in a general sense, 

 somewhat as suggested by Lacroix,* indicating a rock rather than a mineral, 

 consisting essentially of the products of alteration of the magnesian schists, which 



* Minlralogie de France el dc ses Colonies, part I, p. 417. 



