348 THE PENOKEE IRON BEAMING SERIES. 



very irregular, and along this contact line at a nxnnber of places the slates 

 and greenstones are mixed in the same confused manner as in the outcrojj 

 just mentioned. On the ground it is difficult at these places to say at 

 what point tlie Penokee series ends and the overlying eruptives of the 

 Keweenaw series begin. The slates are cut by masses of greenstone which 

 seem to be at times interbedded and at other times in the nature of dikes. 

 These relations are what would be expected along the line of contact of a 

 fragmental series and an overlying eruptive one. 



General character of the rock. — The igneous material of the Penokee series 

 has all unmistakably been at the time of its intrusion one species of rock, 

 diabase. Ocoasionally the diabases vary toward or into a gabbro, but this 

 is exceptional, and all plainly belong together. The degree of alteration is 

 very great at'times, large masses of rock l^eing totally decomposed ; but 

 even the most altered j)hases so frequently retain their diabasic structure 

 and can be traced into a comparatively fresh rock that the conclusion is 

 reached that thev all are altered diabases. The eruptives in the central 

 part of the Iron-bearing member (the ore-i)roducing part) are those which 

 are most altered. The greenstones in the Upi)er-slate member are much 

 fresher upon the whole than those contained in the iron belt ; while occa- 

 sionally there is here found a fresh olivine diabase in which even the olivine 

 is mostly unaltered. As there is every reason to believe that the rocks here 

 found are contemporaneous with the dikes contained in the iron belt (are 

 in fact, in all probability, in many cases but continuations of them), it would 

 seem to follow that the great difference in degree of alteration in the two 

 cases is due to the difference of characters of the rocks in which the dikes 

 are contained. This difference in alteration is interesting in connection with 

 what has previously been said in reference to the origin of the ores. It was 

 noted that the iron-bearing belt (particularly in the part which bears ores) 

 is much more readil)' penetrable by percolating waters than the slate belt. 

 The lower horizon iron rocks have been subject, at least near the present 

 surface, to a long series of changes which have been shown to be due to 

 percolating waters. The ready penetrability of the Iron-bearing member 

 by percolating water has also been favoraVjle to the alteration of the inclosed 

 greenstones. ( )n the other hand, the Upper slate, composed of a series of 



