from Northeastern Minnesota. 179 



An analysis of one of the best examples of these schists, 

 No. 8779, from S.E. \ of S.E. £, Sec. 31, T. 61, K K. 12 W., 

 Minn., yielded Mr. W. R. Melville the following figures : 



:\ T o. 87T9. No. 8784, 



Si0 2 12-35 1-16 



Ti0 2 .... "12 none 



Al o 3 -- '10 1-81 



Fe 2 3 58-68 69-08 



FeO 21-34 27*10 



MnO ._. 1*22 -33 



CaO 1-91 *53 



MgO 4-08 -25 



Na„0 tr. 



P 2 6 -25 -06 



Loss° -19 



Total.. ._. 100-24 100-32 



This is a rock composed exclusively of large plates of the 

 colorless amphibole and large grains of magnetite. The latter 

 mineral was not analyzed, but a rock from near E"o. 8779, con- 

 sisting of almost pure magnetite was analyzed by Mr. Melville, 

 who reports the results entered under No. 8781. Neither the 

 magnetite in 8779 nor that in 8781 contains any notable quan- 

 tity of titanium, and thus they differ markedly from the 

 gabbro magnetites bordering much of the south side of the 

 Mesabe range. 



With respect to the amphibole in this rock it is manifestly 

 impossible to decide positively whether it isgriinerite or actin- 

 olite. If we regard the Fe a 3 as existing in the magnetite, 

 and ascribe to this component also the MnO, half of the MgO 

 and sufficient FeO to satisfy the formula Fe 3 4 , there remain 

 the other oxides, after deducting enough CaO to combine with 

 the P 2 5 in apatite, in the following proportions : 



Si0 2 Fe 2 0, A1 2 3 FeO CaO MgO Total 



71-63 -92 -58 5-80 9*40 11-60 = 99-93 



which are nearer those of actinolite than of any other amphi- 

 bole, although the percentage of Si0 2 is abnormally large. 



One specimen, No. 8783, consists of several alternating light 

 and dark bands. Of the latter it need only be said that their 

 composition is similar to that of the actinolite magnetite schists, 

 i. e. they are composed of large plates of amphibole thickly 

 strewn with grains of magnetite. The dark bands pass gradu- 

 ally into the light ones, the magnetite decreasing in quantity 

 as the borders of the light bands are reached, until in the 

 interiors of these bands all of this mineral has disappeared. 



