468 THE PENOKEE IKON-BEAIJING SERIES. 



of this pile the feldspars have, it is true, in large measure decomposed to 

 chlorite, mica, and quartz. Also the quartzite of the upper part of this for- 

 mation has been indurated by the enlargement of the quartz-grains. The 

 alteration in parts of the Iron-bearing and Upper slate formations has locally 

 been more extensive. But the clastic character of the fragmental belts is 

 usually seen at a glance under the microscope. The pressure to which these 

 rocks have been subject, the amount of which one dares hardly to estimate 

 in figures, has not been sufficient to distort the quartz-grains in an v degree ; 

 nor has it been sufficient to give a schistose structure to the quartz-slates. 

 It is a probable deduction from these facts that the weight alone of any ordi- 

 nary amount of superincumbent rock is not sufficient to develop schistose 

 structure. The schistosity found in the various fragmentals of the Mar- 

 quette, Menominee, and Vermilion lake districts, and in some places in the 

 Eastern area of the Peuokee series, nmst have been developed in connection 

 with the dynamic action of folding to which they have been subjected. 



SECTION III.— CORRELATION. 



Throughout this memoir the terms Archean, Laurentian, Keewatin, 

 Huronian, etc., have been avoided. Certain of these words have been so 

 differently used by different writers that it was thought best to free the dis- 

 cussions involved iii this book from misapprehensions Avliich would result 

 from their use. The structural relations of the series considei'ed are so per- 

 fectly distinct that no evidence or use of names from other localities is 

 necessary in order to make out the succession of belts within the Peuokee 

 district itself, or the relations of the series to one another. Before closing, 

 however, it is necessary that some reference be made to other series, which 

 have been designated by these and other terms, and an indication be given 

 of our judgment as to their proper use, and where the series considered 

 would fall under that usage. 



Equivalency of Penokee series proper tvith Animikie series. — As the ojien- 

 ing step a comparison will be made with the Animikie series (see PI. 

 xxxvii), the equivalency of which with the Penokee series proper is as 

 plain as the equivalency of any two areas of detached rocks in a single 

 geological basin can possibly be in which clear paleontological evidence 

 is lacking. 



