THE ERUPTIVES. 355 



Eruptivcs in the Iron-hi-arinu mrmhcr. — The eruptives contained in the 

 iron formation are mostly in the form of dikes. This is i)articuhirly true of 

 its central part, but in T. 47 N., R. 45 W., Michigan, there are eruptives 

 which are ajjparently interleaved. The great abundance of these dikes in 

 the parts of the iron formation which have been cut by mining, and their 

 relation to each other as well as to the containing formation, are shown 

 by Pis. XXX and xxxi. 



Macroscopically, the dikes vary in extent from a coarse diabase to a 

 very fine gi'ained rock. The fact that the greenstones included in the 

 iron formation are as a whole altered to a much greater extent than the 

 erjiptives elsewhere iii the Penokee-Gogebic series has been noted in a 

 previous chapter. Even those which are very much altered retain the 

 characteristic structiire of a greenstone, although in some cases the decom- 

 ])iisition has gone so far, or else the rocks were originally so fine grained, that 

 they are now aphanitic. The greenstone structure is usually so pronounced 

 that the writer in the field had little doubt as to the real nature of these 

 rocks, even before they were traced into the less altered phases, and of course 

 before a microscopical stud}' of them was made. In several mines the same 

 dike in different places is a comparatively little altered rock, which is prop- 

 ei-ly a diorite, and a completely altered one, which is a typical soapstone. 

 The great Colby dike presJents a good instance of this, its major portion 

 being, however, extremely altered. The color varies, depending upon the 

 degree of alteration, from the dark greenish gray of the ordinary diabase 

 of the Penokee-Grogebic series, through various shades of dark green and 

 light green, to almost snow white. The more common jjhases of the rock 

 are dirty greenish white. All are colored various shades of brown and red 

 upon their surfixce and along the cracks by iron oxides, and oftentimes 

 these stains have penetrated the solid dikes to a considerable distance. As 

 a result of the decomposition the diabases have become very soft, so that 

 they can readily be scratched by the finger nail, and a specimen may be 

 broken to jneces in the hands. This softness has not resulted, however, 

 in making them porous or less compact than -vyhen unaltered. Their non- 

 penetrability by water is clearly shown l)y the fact that the interiors of 

 the dikes are in the main unstained by iron oxide, which has colored red 



