358 THE TENOKEE IKON-BEAEING SERIES, 



that all of the titanium which was originally present in the magnetite re- 

 mains in 'the decomposed rock. The percentage of silica which is taken 

 away is really larger than is indicated by the difference between 1 and 3, 

 since in the latter so large a proporton of water is present. Pure kaolinite, 

 H4Al2Sio09 contains SiOa, 46-50; ALOj, 39-56; ILO, 13-94. Disregarding 

 the small amount of impurities in No. 3, its composition corresponds very 

 closely with this mineral. It seems plain that, as a result of the leaching 

 action to which the lower part of the iron formation is subjected, the dikes — 

 that is, augite-plagioclase-magnetite rocks — are so changed that their compo- 

 sition is very close to that of the mineral kaolinite; although it is possible 

 that this average composition is due to several important minerals rather 

 than to a single one. 



That the diabase dikes high up in the Penokee series are really con- 

 tinuations of the dikes which cut nearly at right angles the underlying iron 

 formation there can hardly be a doubt. The contrast between the two is a 

 striking instance of the influence of environment upon the decomposition 

 of a rock. The diabases inclosed by the impervious Upper slate have 

 been kept in a well preserved condition through the ages which have 

 elapsed since their intrusion, and some of them are remarkably fresh. 

 Other parts of the same dikes in a formation which contains evidences of 

 having been long subject to the action of percolating waters have been 

 completely decomposed. It thus appears that in this case environment has 

 been a far more important element than age in the preservation of the rock. 



Summary. — The Penokee eruptives are of two classes. Dikes cutting 

 the formations, and interbedded sheets, which are probably intrusions of the 

 same age as the dikes. 



The eruptives are diabases, which occasionally pass over into gabbros. 



Diabases in every respect like those of the Penokee series are found 

 both in the Southern complex and in the Keweenaw series. These are all 

 presumably of the same age; that is, Keweenawan. 



The Penokee diabases are in all respects typical rocks of their class. 



In one case a rhombic pyroxene of some interest is present. 



While the diabases of the Upj)er slate member are often quite fresh, 

 these rocks have generally undergone an extensive series of alterations ; 



