46 BULLETIN OF THE 



(91, 92) was observed cutting across the iron-bearing rocks, bending and 

 breaking them at the junction between the two. The " diorite " holds 

 fragments of the iron-bearing rocks iu it, and cuts very irregularly- 

 through the schists. Its intrusive character is distinctly marked by the 

 relation which it holds to the country rock. It renders the rock adja- 

 cent to it magnetic for a little distance from the junction. A section 

 taken from a specimen (92) free from garnets is seen now to be com- 

 posed almost, if not entirely, of secondary minerals, — a confused aggre- 

 gation of greenish hornblende, orthoclase, quartz, and biotite, with 

 traces of magnetic iron. The character of the hornblende is the same 

 as that seen to occur in those rocks whose hornblende is derived from 

 the augite by alteration. 



To the southeast of this locality, not far from the granite, the iron- 

 bearing rocks were again found in contact with a similar garnetiferous 

 " dioritic " dike ; the former being much contorted and broken, the iron 

 being also magnetic near the contact. A tongue of the " diorite " pene- 

 trated the ferruginous rock, and the relations were such that it was 

 evident that the contortion was owing to the intrusion of the "dio- 

 rite." The "diorite" (121) marked on Mr. Brooks's map of Republic 

 Mountain, lying between the quartzite and Smith's Bay, is seen under 

 the microscope to be very much altered, and composed of hornblende, 

 biotite, plagioclase, orthoclase, quartz, and magnetite, with a little hema- 

 tite. Of these minerals, it would seem from their structure and rela- 

 tions, that only the magnetite and part of the feldspar were original 

 constituents. 



The basic intrusive rocks mentioned in the preceding pages have, in 

 general, been regarded as diorite, Mr. A. A. Julien especially doubting 

 the presence of any diabase in the region.* They would pass, according 

 to the ordinary definitions, macroscopically and microscopically, for dio- 

 rite, quartz diorite, diabase, chlorite schist, hornblende schist, etc., yet 

 we regard them all simply as more or less altered forms, according to 

 age and conditions, of rocks that were originally the same in origin, 

 structure, composition, and name, — basalt. Of course, the supposed 

 altered andesite (p. 38) would be an exception. The reasons for so 

 regarding them have been briefly pointed out in this and preceding 

 papers. + Excepting the melaphyr of the New Washington mine, we re- 

 gard them all, then, as the alteration form of basalt known as diabase. 



* Geol. of Mich., II. 42, 193. 



t Proc. Boston Soc. Nat. Hist, XIX. 217-237, 309-316; Bull. Mus. Comp. 

 Zoology, Cambridge, V. 275-287. 



