THE ironbp:aring member. 211 



metallic lustered particles, it I'ails l'i-L!(jueiitly to j^-ive the reddish tran.slu- 

 cency ordinarily de.scriV)ed as a cliaTacteristic of it.' 



Of the three ])rin(;i})al mineral consTitucnts named, tlie quartz, or 

 silica, priiseiits itself in a general way as 'a very minutely crystalline 

 gToun(huass or l)acki;rounil in which the othci- minerals are contained. 

 Only rarely is the crystallization of this (juartz so coarse as indicated in I'l. 

 XXVIII, Fig. 4, in which, however, the individuals are still small, heing 

 greatlv magnified. On the other Iiand, it dwW rarelv sinks to that exces- 

 sive degree of finene.ss which is characteristic of the chalcedonic form of 

 .silica. Still, a few sections have been found in which it does reach this 

 excessively tine condition, and even becomes (|uite amorphous; Init these 

 conditions are, as seen, far more characteristic of the chertv rocks of the 

 type previously described. Since these two ty])es, h(twever, manifestl\- 

 grade into one another, it is evident enough that all of the silica of the 

 three tyi)es is of the same origin; that is to say, is ;d] water de[)osited. 

 Even where lea.st minutely crystalline, as in some of the actinolitic slates, 

 the individuals of cjuartz interlock with one another in such a tashion as 

 to plac(f their deposition in situ (juite beyond (question. Nowhere is there 

 any indication of a fragraental origin for any of the siliceous groundmass. 

 Cases have been noted of the occurrence of genuine fragments of quartz 

 within the nonfragmental mass of the rocks of the first and sec(in<l ty])es, 

 but these fragments are always sharply define<l and wholly distinct in 

 character from the prevalent nonfragmental silica. 



"^I^he magnetite and hematite occur partly in the form of an excessively 

 fine dust contained within tlie (piartz and actinolite imlividiials, ])artl\- in 



' .\ siii;^li' inilividiLiI of lii'iiiatite is clianicterized in tlir tbiii section ordinarily liy a rcddiHh 

 color in transmitted liglit, and laek of metallic Inster in reflected liglil; ''"t wlicii the jndiviilnals are 

 exeeeditiK'.V small, an aft);rej{ate of hematiti! may readily present tlie metallic liisli r, :iihI unless thin- 

 ner than the orilinary thin section is, may at the same time ajipear piMteclly opai|ne in transmitted 

 li;;ht. in which case, if niiiiKlecl with m;i;;netite, its presence isdilhcnit toditeit witli the microscope. 

 In iiriliM' to lest this i|tieslion a inimln'r of thin sections were made of spec n la r lieinalitcs I rum varions 

 places in the lake Superior region. .Some of these, after powdciini;, yielilcd nothin;; at all to the 

 mafjnel, while others contained nmre or less maf;netite interniinfjlcil with the hem.iliti'. In all cases, 

 liowevcr, where the (jrain is sntTicicntly line, it was found that llic licmatitc |irisciilcd the same 

 metallic luster in the reflected lifjht ami o]iacity in tr.ansmitteil liffht as the nia;;nctife. .Many of 

 the.se thin sections, when the ore containi'd no nia^netite whatever, presenti'd in the transmitted 

 Ii;;ht .1 mixliin of led transliiciiil uTid black op,ii|iie material, this o|iai|ne material ^ivingthe metal- 

 lic luster in the reflected liKbl. 



