GEOLOGICAL EXPLORATIONS AND LITEItATriiE. ^5 



change, the author shows that the process inchided also tlie sccoiulary 

 enlargeraent of quartz frag-ineuts, and occasionally of feldspar fragments. 

 The facts which are summarized in this paper are gi\'en in detail in snJise- 

 quent pages of the present volume, of which, in fact, the ]iaper was merely 

 an advanced announcement. 



It should be said that, although these conclusions of Prof Van Ilise in 

 this paper have stood the test of all of our later works, we have never 

 yet been able to trace with certainty to a similar origin completely crystal- 

 line mica-schists which belong to the lower or liasement series. Penokee 

 mica-schists, as also others like them from the Marquette district of Mich- 

 igan and froni the Mississippi valley in the vicinity of Little Falls, are 

 perhaps rather mica-slates than mica-schists; at least they are not highly 

 foliated. The mica-schists, which are associated with the ancient gneisses, 

 may or may not have had a similar origin, so far as our observations 

 have gone. 



IRVTNG (R. D.). Origin of the Ferrusiiions .Scliists and Iron Ores of tliP Lake 

 Superior Region. Am. .Fonr. Sci., 3(1 series, vol. xxxii. ISSC, ])j). :i.').5-l*7l.'. 



Like the preceding paper, this one also is a preliminary statement of 

 results, the better proofs of which n^sidts are elal)orated at length in 

 subsequent pages of the present volume. The paper argues that the 

 original form of the beds of the ii'on-l)earing horizons of the lake Suj)erior 

 region was that of a series of thinly bedded carlionntes interstratified with 

 carbonaceous shaly layers, which were nlso often impregnated by the 

 ferriferous carbonate; that by a process of silicitication these carbonate- 

 bearinji' layers were transformed into various forms of ferruirinous rocks 

 now met with in this region; that the iron removed from the original rock 

 at the time of silicitication jiassed into sohition in the percolating waters, 

 to be redepo.sited in various places as it became further oxidized, thus 

 making ore bodies and various iron-impregnated rocks: that in other ))laces, 

 however, instead of leaching it out more or less comidetelv, the silicifx'ing- 

 waters seem to have decompo.sed the iron carbonate in phice in such m;nmer 

 as to produce the well known actinolitic magnetite-schists of these districts; 

 that the bodies of rich ore have probabl)- had difterent origins in different 

 cases, some having- originated from a direct oxidation in j)lace of the 



