ROGERS, GEOLOGY OF THE CORTLANDT SERIES 37 



of biotite in a norite may generally be taken to indicate this variety. 

 The identification of the hornblende in the presence of hypersthene is 

 difficult, owing to the peculiar luster of the latter on cleavages. Occa- 

 sionally, however, the hornblende occurs in slender black rods, whose 

 long axes are roughly parallel, when it is unmistakable. 



In thin section, the feldspar resembles that of the other norites. The 

 plagioclase is usually a trifle more basic than in the others, being gener- 

 ally either a basic andesine or a labradorite. In one case, microperthite 

 was found, but in another, the plagioclase was almost bytownite. The 

 orthoclase is perhaps less abundant in hornblende norite, though almost 

 always present. The hypersthene is usually deep in color and pleochro- 

 ism. The hornblende seldom resembles that of the diorites, being gener- 

 ally coarser and of the brownish variety. The greenish tint is usually 

 present, however, though faint. The pleochroism is good: X dark 

 brownish green, Y dark brown, and Z yellow, with the usual absorption 

 formula. The alteration is to chlorite. Biotite and augite are not in- 

 frequently present in relatively small amount. Apatite is often found 

 in great abundance, size and perfection, and ilmenite is also common in 

 large irregular masses. Titanite is rarely present in moderate amounts. 



The order of crystallization exhibited in this rock is often peculiar. 

 In many cases, the hypersthene, biotite, augite and even plagioclase seem 

 to have preceded the amphibole, which then occurs in large irregular 

 plates containing all of the other minerals excepting orthoclase. The 

 structure then approaches the poikilitic, this term having been coined by 

 Williams in fact to describe a similar feature in the hornblende of the 

 hornblende pyroxenites of Stony Point. 30 In other cases, the plagio- 

 clases are outlined by irregular streaks of ilmenite, so that in two respects 

 a somewhat extraordinary reversal seems to have taken place. 



From the frequent presence of augite and biotite in small amounts, 

 the hornblende norite seems to have its most pronounced relationship 

 with biotite augite norite. It is, however, most frequently in contact 

 with this rock. A larger increment of biotite than is usually seen re- 

 sults in the less common biotite hornblende norite. In the smaller area, 

 however, it appears to grade into the diorite by loss of hypersthene, the 

 variety of hornblende being the same in both rocks. 



Biotite Hornblende Norite 



An area of hornblende norite which carries biotite in addition, lies 

 just southwest of Dickerson Hill. It may be placed in the same cate- 



30 Amer. Jour. Sci., (3), XXXI, 30. 1S86. 



