Lake Superior Iron Ores.—S purr. 345 
and Renard* from off the coast of Australia, afforded less 
than one per cent. This being the case, our two analyses of 
the Mesabi mineral, with their .41 and .31 per cent potash, 
come far closer as regards potash, to the glauconites low in 
potash above quoted, than these do to the glauconites high in 
potash. Moreover, Murray and Renard concluded that by a 
study of the ancient rocks near the coast where the glauconite 
forms “it is possible to suggest, with a considerable degree ot 
certainty, the relative abundance of the potash in the deposites 
where the glauconite is forming.’”’ For since the glauconite 
derives its constituents especially from the “debris of granite, 
gneiss, mica-schists, and other ancient rocks’ which “must 
give birth by their decomposition to potassium, derived from 
the orthoclase and the white mica of the gneisses and the 
granites” the relative abundance of potash in these rocks will 
roughly correspond to that in the glauconites. Now the land 
surface at the time of the deposition of the Mesabi iron-bear- 
ing formation must have consisted of the complex of pre-Ani- 
mikie granites, gneisses, and schists. Ordinary granites gen- 
erally contain 4 to 8 per cent of potash, but several analyses} 
from the region under consideration} show a small amount of 
this substance. Of three analyses, one showed 2.80 per cent 
K,O, another 1.68, and a third .71. The schist seems still 
poorer; an analysis of green schist from Tower showed .30 
per cent potash, and another from the falls of the Kawishiwi 
.27 per cent. These are the only analyses the writer has access 
to at the present moment. On the other hand, the green 
schists contain plenty of iron (one of the above mentioned 
analyses shows 9.87 per cent, the other 19.23 per cent iron 
oxide) ; so it seems only natural that the ferrous silicate should 
form in abundance, under the influence of organic matter, in 
the ocean into which the mud from these rocks was washed; 
and that it should be destitute of potash. 
Of more importance, perhaps, than the exact chemical 
composition of a mineral are its physical qualities. Thus the 
amphiboles are related more by form and optical characters 
than by chemical composition. The same is true of pyroxene, 
* Reports, Challenger Expedition, vol. Deep Sea Deposits, p. 387. 
+ U.S. GRANT, 21st Ann. Rep., Minn. Geol. und Nat. Hist. Surv., pp.+1-44. 
Also AM, GEOL., June, 1893, p. 385. 
t The first is from Kekequabic lake; the second from the Kawishiwi river; 
the third from Saganaga lake. 
