r72 NEW YORK STATE MUSEUM 



It is to be noted that Kolderup's quartz norites are more 

 typical for tlie rock than the one analyzed from the Adirondacks, 

 that approaching anorthosite more closely and being more like 

 his anorthoeite norite in many respects. His quartz norites show 

 lOfc or more of quartz as against the 5/^ of the Rand hill rock. 

 But, as has been stated, the larger part of the Band hill rock is 

 more quartzose than the specimen analyzed and would in all 

 probability approach his quartz norite very closely. 



The chemical differences between the anorthosite gabbro of 

 analysis 3 and the anorthosites of 4 and 5 of the original table 

 are flight, 3 showing diminished silica, alumina and soda, and 

 increased iron and magnesia; they suffice however to cause a drop 

 in the feldspar content from over 90'fc in the anorthosite to 70^ 

 in the anorthosite gabbro. In all these anorthoeitic rocks part 

 of the potash is in the labradorite, replacing a certain amount of 

 soda. Analyses of this feldspar always show it, and, in calculat- 

 ing the rock analyses, it is necessary to assume that part of the 

 calculated orthoclase goes with the albite to form labradorite, 

 in order to bring about agreement between the computation and 

 the observed optical properties of the feldspar. 



The rock analyzed in column 6 ha^ the appearance of an inter- 

 mediate rock in the hand specimen, the feldspar augen resem- 

 bling labradorite, and being sometimes iridescent, the granular 

 portion having the look of augite syenite. Cleavage fragments 

 from the augen give extinctions of — 5° on 001 and — 19° on 010, 

 and hence are close to labradorite, Ab^ An^. But the granular 

 feldspar is in part microperthite, and in part an acid plagioclase. 

 The alkali percentage is abnormally high for so basic a rock. 

 The total bases bear a very high ratio to the silica and alumina, 

 and the considerable alteration of the augite to a chloritic aggre- 

 gate renders attemfpts at calculation of the mineral percentages 

 hazardous. The rock is approximately composed of orthoclase 

 20;?^, albite 44;^, anorthite 11^, magnetite 4;^, and the remainder 

 of augite and garnet in the ratio of 2 to 1, including a little horn- 

 blende, apatite and quartz, the latter only as a by-product of 

 garnet formation. In its high alkali percentage and consequent 



