190 THE PENOKEE IRON-BEARING SERIES. 



have no means of determining accurately. Throughout much of tliis area, 

 moreover, the uppermost portion of the iron beh, is missing, l)ut how much 

 is gone tliere is no means of telhng. It is only })0.ssible to say in a general 

 way tlint it would seem tliat in the eastern i)art of the formation its thick- 

 ness Miiiv he vcrv niatcri;dly increased. 



ircticfiil jicfni(/r((plii(ti/ ihiuditcr. — Three main ty{)es of rock make np 

 the Iron-lx'iu-ing member. Between these three types .there are various 

 gradation phases, while each main type ])resents itself in a number of 

 forms l)etween which there are minor differences. In certain rare instances 

 a little detrital material has been introduced during the original deposition 

 of these rocks, but ordinarily this is completely lacking. The three tyjies 

 referred to may be briefly characterized as (1) .sJatji and nftcii chert i/ iron 

 carbonate, (2) ferruginous dates and ferruginous cherts, and (3) actii/olitic and 

 magnetitic slates. 



The first type is a very well marked one, and is ])resent in very con- 

 siderable thickness. It is characterized by the invariable presence of iron 

 carbonate as a chief constituent. In some cases the iron carbonate con- 

 stitutes the only important mineral in the rock, but usually it is mingled 

 with more or less of calcium carbonate, magnesimn carl)onate and cherty 

 silica, the latter ranging in character from minutely crystalline to amorphous. 

 Other minor ingredients, one or more of which may l)e present, are hema- 

 tite, limonite or other brown iron oxide, magnetite, carbonaceous or graph- 

 itic matter, iron pyrites, a chloritic or viriditic ingredient, an excessively 

 fine clayey substance, and, ^■ery rarely, j)ieces of a fragmental (|uartz. In 

 textiu-e these rocks are commonly earthy and aphanitic, but in some cases 

 the iron carbonate is sufficiently coarsely crystalline for the cleavage sur- 

 face of the minute crystals of siderite to be perceptible to the naked eye. 

 There is very commonly a regular and thin lamination produced by aii 

 alternation of lighter and darker gray shades of the carbonate. Frequently, 

 however, the gray carbouiite is interlaminateil with seams of ])lack graphitic 

 matter, red jasper, red licinatitc, l)lack flint, greenish l)lack or viriditic car- 

 bonate, or with seams of carbonate in which magnt'tite ])articles are ])artic- 

 nhn-l\- abundant. Wliilc the pliity or thinly stratiform liabit is very char- 

 acteristic of these rocks, tliesc lamina" often l)ecome irregular, presenting 



