AUOHEAN OF MICHIGAMME MOUNTAIN AREA. 429 



In the Arclleaii areas granites of two ages have been found, the 

 younger in the form of narrow dikes. Basic igneous rocks, also in dike 

 form, are rather abundant. One of these under the microscope j^roves to 

 be a but little altered diabase, in which the augite is almost intact. These 

 acid and basic intrusions are probably connected with the surface flows of 

 like character which are so abundant at the Mansfield horizon along the 

 Fence River. 



Of much interest is the occurrence of a small mass of quartz-porphyry 

 in contact with the Archean, and below the lowest Algonkian sedimentary 

 formation. The locality is in sec. 21, T.'44 N., R. 31 W., in the southeast 

 quadrant of the Archean oval. The upper surface of contact of this sheet 

 with the lowest sediments is covered, and hence it is not entirely certain 

 whether it is intrusive or extrusive, and therefore whether it belongs to 

 Archean or Algonkian time. The general relations, however, appear to 

 indicate that it is a surface flow which suffered erosion before the deposition 

 of the basal Algonkian member, and is therefore to be classed with the 

 Archean. The exposure is 250 feet long by 100 broad. The rock consists 

 of a very finely granular matrix of a warm gray color, through which 

 are sprinkled quite uniformly little grains of blue quartz, and larger rounded 

 grains of pink feldspar. Flakes of biotite are scattered through the ground- 

 mass and coat the cleavage surfaces, which are developed in two distinct 

 systems, intersecting at an angle of about 10°. Immediately below the 

 porphyry is coarse porphyritic granite, sheeted in waving surfaces parallel 

 to the contact, which dips eastward about 40°. The lower portion of the 

 porphyry contains a number of fragments of the underlying granite, one of 

 which is over 4 feet in length. 



Under the microscope the groundmass is a fine-grained crystalline 

 aggregate of quartz, greenish biotite, and a little feldspar. The quartz 

 phenocrysts are beautifully corroded, and have the characteristic bipyrami- 

 dal form, while the feldspars are extensively altered to biotite, sericite, and 

 quartz. 



Biotite-gneisses related to this porphyry in external appearance occur 

 among the Archean outcrops inclosed in the "B" line of magnetic attraction 

 in sec. 7, T. 45 N., R. 30 W., and may be described here for comparison. 

 They are dark-colored, fine-grained rocks, which weather to light pink. 

 They are eminently schistose, and the cleavage surfaces are coated with 



