PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 471 



Melaphyre.] 



rather fine-grained, opaque, reddened feldspar and some quartz with magnetite, apatite 

 and the usual alteration products. The augite does not appear to be idiomorphic, 

 although its alteration would more or less obscure any idiomorphic tendency which 

 it might have had originally. The feldspar in places shows that it is older than the 

 augite. 



One section examined. 



Age. Cabotian. 



Remarks. This specimen, as well as others (Nos. 644 and 648) in this general 

 vicinity (and for that matter many in northeastern Minnesota), can be explained by 

 assuming that the basic gabbro magma absorbed from the surrounding rocks more 

 or less acid material. In the specimen here described the porphyritic feldspars, which 

 appear to be lime-soda feldspars, although from their altered condition this point was 

 not determined with certainty, and the augite, i. e., the more basic minerals, crys- 

 tallized out first as porphyritic crystals, and the rest of the rock the more acid part 

 solidified later under different conditions as acid feldspar and quartz. 



In this connection it is of interest to note that it appears to be only the acid 

 feldspar which alters to the opaque red material so common in the red rocks of the 

 north shore of lake Superior, while the more basic feldspar does not alter in this 

 manner. u. s. G. 



No. 650. MELAPHYRE. 



"This is so abundant that it must be in place near, but cannot be seen.'' Evidently from approximately 

 the same place as No. 649. 



Ref. Annual Report, x, page 68; Bulletin ii, pages 101, 102. 



A dark -gray rock, apparently granitoid in texture, composed of gray feld- 

 spar and much black material. 



Mic. The following is the description of this rock written by M. E. Wadsworth:* 

 "Is a dark grayish brown, somewhat porphyritic rock, whose section is composed 

 of lath-shaped and tabular feldspars, with augite, olivine, quartz and biotite, lying in 

 a brownish groundmass. The feldspar is both orthoclase and plagioclase, while part 

 is secondary. The groundmass is now the replacement of a former basaltic ground- 

 mass, together with part of its porphyritically inclosed minerals. At the present 

 time this groundmass is the same as that of many quartz-porphyries, and is composed 

 of a confused aggregation of quartz, feldspar, ferrite, magnetite, microliths, mica 

 scales, etc. The section, in certain portions, with its larger secondary quartz, could 

 well pass for a felsyte or quartz-porphyry. The augite is brownish and shows a 

 strong tendency to alter into pale yellowish grains and irregular crystals associated 

 with feldspar and quartz. This is seen most commonly in the vicinity of magnetite 

 masses. The pale yellowish grains and crystals show the general characters of 



Bulletin ii, pp. 101, 102. 



