134 THE PENOKEE IRON-BEARING SERIES. 



One of the most notable peculiarities of this cherty rock is its tendency 

 to assume a brecciated form, in which angular pieces of the chert, ranging 

 from microscopic sizes to fragments two or three inches across, are buried 

 in a chert of a character wholly similar to that of the fragments or differing 

 from them only in carrying a small quantity of other ingredients, such as 

 magnetite and chlorite. These brecciated phases occur at times wholly 

 within the horizon of the Cherty limestone member itself, and in other cases 

 are very near to its junction to the overlying slate, when it is not always 

 easy to draw the line between the two members. 



Change to the overli/iiif/ Quartz-slate. — As already indicated and subse- 

 quently further explained, the limestone or basal member of the sedi- 

 mentary series is directly overlain by a very considerable thickness of com- 

 pletel)' fragmental rocks, whose main constituent is quartz, which mineral 

 is, however, accompanied by a large proportion of feldspar frag-ments and 

 by various alteration derivatives from the feldsj)ars. llie change from the 

 limestone member to this quartz-slate is very sharp, the fragmental rock 

 often carrying at its base pieces from the cherty material belonging directly 

 beneath it. As this detritus in the quartz-slate is precisely like the material 

 of the cherty limestone as it now exists, there must have been a considerable 

 lapse of time between the deposition of the two formations. 



Tahuhtfioii of petrographical observations. — Althougli, in the ])resent de- 

 velopment of the knowledge ot petrograj)hy, it is rather unusual to include 

 a description of individual sections in a volume like this, it is g'iven in this 

 and succeeding chaptei's because the essential unity and continuity of each 

 formation, as well as the contrast between it and the following one, thus 

 appear with a clearness which can be enforced in no other way. 



This detailed observation is desirable for the further reason that, as 

 explained in the Preface, the Penokee district is dealt with in greater detail 

 than is intended with any subsequent iron-bearing district, the reason for 

 the great elaboration being that this is the first of the iron-producing 

 areas of lake Superior in which the geology has been fully worked out. 



The numbers of specimens and slides are usually those of the collec- 

 tion of the lake Superior division. Specimens with Wr. after the numbers 

 are from the collection of the late Mr. Chas. E. Wright. Specimens with 



