REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1900 r39 



this sort cf feldspar usually accompanies increasing content of 

 ferro-magnesian silicates. 



But the interesting matter in this section is the dike in the 140 

 yard cut (954B), interesting because it brings out clearly the rela- 

 tionship of the syenitic rocks described from- the preceding sec- 

 tions, to the anorthoisite, since we have here a strictly intermedi- 

 ate rock, resembling precisely those whose feldspar is wholly of 

 the anorthoclase type yet with much plagioclase present in 



addition. 



Exposures about Brandon 



The little hamlet of Brandon on the New York and Ottawa rail- 

 road is included in the Rockefeller purchase and is now in proc- 

 ess of demolition. It lies close to the anorthosite boundary, 

 but exposures are not as frequent nor as good as could be wished. 

 Around the St Regis lakes and thence down the river toward 

 Brandon the exposed rock is of typical anorthosite, the feldspars 

 constituting from 95^ to 99^ of the rock and showing extinction 

 maxima of from 22° to 26° in the various slides. Just about' 

 Brandon it varies from the type, though still consisting mainly of 

 feldspar. The proportion of untwinned feldspar has however 

 considerably increased, and some of the blotchy intergrowths are 

 evident, indicating that anorthoclase is becoming prominent. 



A hill J mile north of Brandon is constituted of amphibolitic 

 and granitic gneisses, and Buck mountain, 2 miles to the west, 

 is composed of the same rocks, showing that at Brandon we are 

 near the anorthosite edge. Yet at Indian rock, a small isolated 

 exposure on the river 2 miles northwest of Brandon and lying 

 within the gneiss zone just mentioned, the rock is again anortho- 

 site (713). It is wholly gneissoid, and in addition to the feldspar 

 contains some 5^ to 10;:^ of hornblende and magnetite, the horn- 

 blende commencing to alter to biotite. The major part of the 

 feldspar shows albite, twinning with 21° maximum extinction. 

 Tliere is also some non-twinned feldspar, which is probably 

 anorthoclase. From its situation this rock is regarded as a prob- 

 able offshoot from the main mass, likely of the nature of a large 

 dike. It is cut by two dikes, one of fine grained and the other 

 of coarse red granite. 



