r40 NEW YORK STATE MUSEUM 



To the southwest of Brandon, in township 16, the most westerly 

 anorthosite outcrops are those at the old Ducey lumber camp. 

 Here the rock (941) is much crushed, though abundant labradorite 

 augen remain. Minerals other than the feldspar make some 15^ 

 of the rock. These are as usual hypersthene, augite, hornblende, 

 magnetite, apatite, zircon and titanite, with a little occasional 

 pyrite. Hypersthene here is unusually prominent for an Adiron- 

 dack anorthosite, much exceeding in quantity the augite and 

 hornblende together, so that the rock is an anorthosite norite 

 rather than an anorthosite gabbro. The feldspar is nearly all 

 twinned and apparently all labradorite. This rock differs from the 

 usual border rock in that the increase in amount of dark silicates 

 is not accompanied by increasing acidity of feldspar. It is more 

 nearly like the rock from the last cut in the Saranac-Lake Clear 

 section (984) than any other, though not quite so basic as that, 

 the feldspar extinctions only reaching 24°. 



General summary of previous sections 



The previous detailed observations are sufficient to establish 

 the fact that there has been some slight differentiation in the 

 anorthosite mass since reaching its present resting place, the gen- 

 eral result being that the rock becomes an anorthosite gabbro in 

 its outer parts, sometimes becoming more basic than the general 

 mass of the rock, but in general retaining about the same silica 

 percentage, increase in the amount of basic minerals being ac- 

 companied by increasing acidity of the feldspar; that the final 

 result of the differentiation in this direction is the production 

 of a rock of gabbroic appearance and with large content of ferro- 

 magnesian silicates, in which the feldspar is no longer plagio- 

 clase but anorthoclase, so that the rock can no longer be classed 

 with the gabbros; that in considerable part the latter rock solidi- 

 fied slightly later than the main mass and was injected into it in 

 the form of dikes, which are most abundant about the edges of 

 the mass but also occur well within it. It further appears that 

 away from the periphery the anorthosite is quite homogeneous 

 in composition. Occasional bands of gabbro cut it, and occasion- 

 ally there is a local enrichment in ferro-magnesian silicates, but 



