60 BULLETIN OF, THE 



interstitial material. The quartz contains microlites and fluid inclu- 

 sions. 



The rock through which the " diorite " (page 43, No. 232) passes, is a 

 highly indurated feldspathic sandstone (233), in which the feldspar pre- 

 dominates greatly over the quartz. It has a greenish, compact, felsitic 

 base, holding grains of quartz. This would easily pass for a quartz por- 

 phyry or felsite, with those who advocate the passage of sedimentary 

 rocks into felsite. It is best classified as a porodite* Microscopically 

 it is seen to be composed of fragments of feldspar with some quartz 

 grains. The feldspar is decomposed greatly, forming micaceous scales, 

 but shows in some cases its triclinic character. It was most probably 

 formed from the detritus of a granite. 



Potsdam Sandstone. 



The sandstones at Marquette resting upon the azoic schists are in 

 the upper portions fine-grained (14, 15), but below they become con- 

 glomeritic. The coarser sandstone (12 a, 13) is composed principally 

 of quartz and feldspar ; the feldspar is the pinkish variety' belonging to 

 the azoic granites in tiie vicinity. Many of the quartz grains are seen 

 to be crystals with unworn facets ; it is therefore probable that they 

 came from veins, and the sandstone making was quite rapid. The other 

 sources of the quartz were probably the granites and quartzites un- 

 derlying them. Coarse pebbles occur in portions of the rock belonging 

 to the adjacent formations : quartzite, ferruginous quartzite, argillite, 

 chlorite, "diorite," etc. (16, 17, 18, 19, 20, 21, 22, 23, 24, 25). The 

 inclosed pebliles were evidently when deposited nearly, if not quite, in 

 the same condition as they and the rocks from which they were derived 

 are to-day ; as was pointed out by Foster and Whitney, South of the 

 Carp River, in the locality figured by Messrs. Foster and Whitney, the 

 sandstone strata are seen to abut against and overlie the vertical edges 

 of the quartzite (29, 30, 31, 32, 33, 34). The dip was about S. 20° W. 

 1G° to 18°. The Sandstone at Presque Isle contains the same materials 

 as that south of Marquette, but has suff'ered a local modification described 

 in connection with the peridotite of that point; — to which vve now pass. 



Peridotite and Serpentine. 



The chief rock at Presque Isle, north of Dead River, is a peridotic 

 one, composed of olivine, enstatite, and diallage, which is the composition 



• Bidl. Mus. Comp. ZoUlogy, V. 280. 



