CHAKACTERISTICS OF THE BASIC GABBROS. 217 



added some brief notes on the microscopic characters. Dr Leeds also 

 describes the macroscopic characters. 



Occurrence and Petrography of the basic Gabbros. 



The more basic gabbros constitute the smaller outlying intrusions and 

 minor portions of the main ridges, which consist chiefly of anorthosite. 

 The best exposure of all is on the lake shore just north of Port Henry, 

 Two great intrusions have come up through the white limestone : one a 

 quarter of a mile above the old Bay State furnaces and one under the 

 Cheever ore bed. Others are met in Moose mountain, in the northwest 

 corner of Crown Point township, where they contain titaniferous magne- 

 tites, which have been opened up by prospect holes ; still others in fine 

 exposure on a hill (locally called Ledge hill) about three miles due west 

 of Westport village, where they also contain notable titaniferous ore beds ; 

 and again as the av all-rock of the old and quite large Split Rock mine, 

 four or five miles north of Westport and fronting the lake. A number 

 of smaller outcrops could also be added, but the above are the best. 



These gabbros are rather dark basic rocks, with an ever present tendency 

 on the part of the plagioclase in specimens, not too metamorphosed, to 

 develop a coarse ophitic structure. The broad rods of the feldspar are 

 quite conspicuous, while between them are the dark silicates. The pla- 

 gioclaSe is oftenest greenish in the hand specimens from the innumerable 

 inclusions which range themselves toward the center of the twin lamella?, 

 as shown in figure 2. They render the feldspar practically opaque. 

 They do not appear to be alteration products, but with high magnifying 

 powers are colorless or faint green and rounded. They. extinguish with 

 the feldspar, by whose optical beliavior they appear overcome. They can 

 only be studied where somewhat dispersed toward the edges of crystals. 

 The feldspar is apparently labradorite. Extinctions in the fresh, clear 

 rims of crystals are prevailingly o°-7°. Where noted at 20°-25° the 

 emergence of a positive optic axis could be detected. The abundance of 

 inclusions neutralizes any specific gravity determinations. In addition 

 to the faint green inclusions one sees minute crystals of hornblende, and 

 jjrobably also of pyroxene, ranged in regular ranks, and reproducing 

 familiar phenomena. Dark, dusty dots have been noticed by many 

 writers on gabbros and are set forth at length in petrographic text-books. 

 In this connection, those described by G. H. Williams from the Balti- 

 more* gabbros and the norites of the Cortlandf series, near Peekskill, 

 may be mentioned, and also those described by Adams from the MorinJ 



*G. H. Williams: Bulletin 28, U. S. Geol. Survey, 188G, p. 21. 



t Idem. : Am. Jour. Sf^ience, February, 1887, p. 141. 



X F. D. Adams : Neues Jalirbuch, Beil. Band viii, 1893, p. 435. 



