THE IKON-BEARING MEMBER. 209 



the areas must be believed U\ be real tragi ueuts which iiave formed either 

 in situ l)y an actual local brecciation of the rock caused liy chemical or 

 mechanical action, or they may be considered as liaving- been dciivcd tVuni 

 the immeiliatcly underlyino- layer of nonfra^inental rock by a temporarv 

 mingling of fraj>incntal and nonfragmental de[)osition. That the fornu'r is 

 tlie true explanation of their origin we have little doubt. 



It is exceedingly difHcult to convey in any general description a good 

 idea of the multitudinous phases })resented b}^ this second type of rock, but 

 some further and more accurate conception may Ije detained by an exami- 

 nation of the accompanying carefully descril)ed plates and study of the 

 detailed descri])tions of individual sections given ]>elow. 



In connection with the second ty})e of rock, it is necessarv to jdhule to 

 some peculiar phases which reseinl»le on the one liand the l)recciated rocks 

 described above, and upon the other the true fragmental rocks of the 

 miderlying Quartz-slate member. In them there has cvidciitlv been a 

 mingling of mechanical and chemical sedimentation. Thc\' contain simple 

 grains of (juartz which have been enlarged, the fragmental cliai-acter of 

 which can not l)e doul)ted. These .same sections also contain rounded (diert 

 areas, which include sometimes little iron oxide, sometimes abundant red 

 hematite, making them jasper, and sometimes lioth hematite and magnetite. 

 Often the areas containing little or no iron oxide and those containing it 

 abmidantly are in juxtaposition. Simple fragmental grains of (piartz and 

 the chert areas are generally arranged with their longer axes in a, common 

 direction. The fragmental character of the simple (piartzcs, the likeness to 

 them in form of the chert areas, and their arrangement with l(»nger axes in 

 a common direction, are almost conclusive jiroof that these parts of the rocks 

 are gemiine mechanical sediments. The fragments are cemented by a 

 matrix, which is a ferruginous chert in every res{)ect siiyilar to the matrices 

 of the ferruginous cherts of the second type of rock. 



These rare occurrences of semifragmental rocks are inttn'stratihed 

 with others which are typical forms of the Iron-bearing member. The 

 meaning of this interlamination of the two classes of sediments will be con- 

 sidered later. 



MON XIX 14 



