REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1900 r45 



Soon after passing Halfway brook, and the west line of the 

 Cornell forestry tract, the firet anorthosite is seen along- the road 

 (935), and a larg'e ledge appears in the woods a short distance 

 to the north. The rock is a medium coarse anorthosite with a 

 considerable dark silicate content, being distinctly of the anortho- 

 site gabbro type. The exposure by the road is merely a smooth 

 glaciated surface, from which it is exceedingly difficult to pro- 

 cure specimens. But it is a most interesting exposure, exhibit- 

 ing the anorthosite as all cut up by a finer grained rock, which 

 is unmistakably younger and intrusiye into the anorthosite. It 

 is thoroughly continuous and separates the anorthosite into great 

 horses. This rock strongly resembles the usual augite syenite 

 and proves to be of that type, somewhat more basic than the 

 usual ruil^f the rock, though quite comparable in that respect 

 with the rock from the first cut along the railway north of Tup- 

 per lake (906) and with that from Colby pond (805iA). It holds 

 uncrushed feldspar augen rather more numerously than the 

 usual syenite and of larger size, though quite comparable with 

 some phases of the syenite. It is quite certainly the same rock 

 as that already described, but here^ instead of grading into the 

 anorthosite or cutting it in single narrow dikes, it has invaded it 

 in force, wedging asunder great blocks of the anorthosiite, which 

 it surrounds. An analysis of this rock appears on a later page. 



Nearly a mile farther east the road takes a curve to the south 

 around the end of a massive rook ridge^ showing outcrops by the 

 road (937) and also a short distance back In the woods (936). The 

 former exposures are of anorthosite norite, rather than gabbro, 

 with much variation in the dark silicate content and in the degree 

 of granulation. They are cut by a large dike of a curious, dark 

 gray porphyry with reddish feldspar phenocrysts, which seems 

 of considerably later date and of syenitic make-np. Such dikes 

 have also been found in other parts of the region cutting the 

 anorthosite. They may possibly be nearly contemporaneous with 

 the hyperite gabbros, forming their complementary types. They 

 have been somewhat metamorphosed and are earlier than the 

 syenite porphyries associated with the diabases. A similar dike 



