GEOLOGY OF THE NORTHERN ADIRONDACK REGION 335 



anorthosite norite in many respects. His quartz norites show 

 10$ or more of quartz as against the 5$ of the Rand hill rock. 

 But, as has been stated, the larger part of the Rand 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 slight, 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$ in the anorthosite to 70$ 

 in the anorthosite gabbro. In all these anorthositic 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 has 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 x 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 attempts 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 

 feldspars, the rock distinctly approaches the syenites, though its 

 silica percentage remains that of the normal anorthosite. 



The gabbro of column 7 is a most interesting rock. Its occur- 

 rence with, and as a differentiation product of an augite syenite 

 body, of which it must be regarded as a basic phase rather than 

 as a true gabbro, and its intermediate position chemically between 



