THE IRON-BEARING MEMBER. 253 



ability in the statement made, that this material was deposited simi;lta- 

 neously with the iron carbonate with which it is so closely associated. 



The fprrugmous slates. — The microscopical description ofthelirst phase 

 of the second type of' rock, the ferrupinons slates (pp. 203-205) has so 

 fully indicated its origin that but little more need he said here than to 

 bring together the actual facts of obserxation. Into the genesis of this 

 phase of rock little or no theory enters, as all of the stages of its growth 

 have been seen. The only conclusion which goes beyond the observed 

 facts is the comparatively safe one as to the nature of those exposures 

 which cannot be directly traced into the cherty carbonates. The more 

 important of the facts which bear upon their origin are as follows: The 

 stratiHcation of these slates is precisely like that of the original carbonates; 

 the mineral composition of the two rocks is the same, except that iron oxide 

 in the ferruginous slates takes the place of siderite; the change from sider- 

 ite to tlie various iron oxides of the slate has been repeatedly noted in all its 

 phases in thin section and is everywhere foiuid in the slides when in the 

 tield the change from the cherty carbonate to the ferruginous slate could 

 be traced. The background of the slates is at times more coarsely crystal- 

 line than in the cherty carbonates ; also, the rocks are cut by veinlets of 

 silica to a greater extent. These differences imply a partial rearrange- 

 ment of the silica which they originally contained, and also perhaps the 

 introduction of a small additional amount of silica. The quantity contained 

 is not materially greater than in tlie unaltered clierty carbonates, and it 

 seems a suthcient explanation of its variation in cliaracter to suppose that it 

 is mostly due to a rearrangement; that is, the silica has been taken into 

 solution to some extent and subsequently cr3-stallized (other silica jicrliaps 

 at the same time being added), thus becoming more pert'ectly quartzose and 

 forming the veins which cut across the lamin* of the rocks. Tlu' only 

 chemical change implied in the above transformation of the chertv carbon- 

 ates into the ferruginous slates is the deconi])osition of the iron carbonate 

 and the peroxidation of the iron which it contained. In the transition 

 forms which occur l)etween these slates and the feiruginous cherts there 

 have been doulitless other chemical changes, but tliev can best be consid- 

 ered in connection with the ongin of this rock. 



