w. s. 



Bayley — Modes of Pigeon Point, Minnesota. 



57 



characteristic peculiarities of the groundmass and its porphy- 

 ritic ingredients as exhibited by several thin sections. 



The microscopical characteristics of both the porphyritic and 

 the granular varieties of this red rock indicate the probability of 

 the identity of the two. Although the most typical quartz- 

 porphyry is quite different in structure from the typical gran- 

 ular variety, all gradations between the two types can be recog- 

 nized. Their mineralogical L 

 composition is the same, 

 and, as will be shown later, 

 their chemical composition 

 is identical. There can be 

 little doubt that the por- 

 phyritic variety is a true 

 eruptive rock. It presents 

 all the features of Rosen- 

 busch's* " Vogesen grano- 

 phyres." Since the granular 

 varieties are so similar to 

 this, it is also probable that 

 these also are eruptive. No 

 trace of fragmental struc- 

 ture could be detected in 

 any one of them, nor is there any field evidence that they 

 are altered fragmentals. All the field relations seem to point 

 to the original character of the rocks. They occur in dykes 

 and veins intersecting other rocks, and the contact between 

 them and the quartzites which they cut, is sometimes clearly 

 seen. It must be confessed, however, that without microscop- 

 ical and chemical evidence of the identity of these rocks with 

 the quartz-porphyry their true nature would be difficult to dis- 

 cover from the field relations alone. A more careful examina- 

 tion of the structure of the point than has thus far been possi- 

 ble, will probably reveal facts which will place beyond doubt 

 the conclusions reached by the microscopical examination. 



The quartz-porphyries are very similar in macroscopic and 

 microscopic appearance to the Keweenawan quartz-porphyries 

 described in Irving, f as flows in the copper-bearing rocks on 

 both sides of Lake Superior. The granular red rock ap- 

 proaches more nearly this author's augite-syenites, though the 

 best developed and most characteristic augite-syenites are more 

 nearly allied to the third phase of the red rock. The rock of 

 Brick Island, which is classed by Irving among the augite- 

 syenites agrees in most of its minute features with the rock 

 described above as the most prevalent type of the red rock on 

 Pigeon Point, as Irving;}: himself states. 



* Die Steiger Schiefer, etc. Strassburg, 1876. 

 f Copper-BeariDg Rocks, p. 95. 



% L. c, p. 369. 



