444 Williams — Gabbros and Diorites of the Cortlandt Series. 



I have as yet been unable to secure a complete analysis of 

 this rock, but an average of four determinations of its silica 

 gives 53 94 per cent. This is sufficient of itself to establish the 

 dioritic nature of the rock. 



The feldspathic constituent varies considerably in composi- 

 tion as may be seen from the different extinction - angles 

 occurring even in the same individual ; nevertheless a number 

 of specific gravity determinations, lying between 2 -67 and 2*648, 

 show that the mineral belongs to the oligoclase-andesine 

 series. Some of this plagioclase is notable for being almost 

 free from twinning striation. This is especially true for section 

 No. 87, from Stony Point, whose feldspar was particularly 

 studied. In other cases the striation is finely displayed, not 

 infrequently according to both the albite and pericline laws. A 

 zonal structure is common, and the delicate inclusions described 

 in detail in the feldspars of the norites,* are sometimes 

 abundant and sometimes absent. 



The mica, which constitutes the only other essential constit- 

 uent of these rocks, is a biotite very rich in iron. Its absorp- 

 tion is intense and basal sections are only translucent when very 

 thin. Their color is then a greenish-brown. The optical angle 

 is so small that it is impossible to determine whether the 

 mineral is anomite or meroxene. The mica contains no 

 other inclusions than magnetite grains and apatite needles. 

 None of the pleochroic aureoles so common in the granites were 

 observed. 



The presence of .orthoclase was not positively substantiated 

 in these rocks. An unstriated plagioclase might easily be 

 mistaken for orthoclase. 



Quartz occurs sparingly in grains, which, from their allotrio- 

 morphous character, were evidently the last product of 

 crystallization. These are frequently penetrated in every 

 direction by the minute and indeterminate black needles 

 mentioned by Hawes, Rosenbusch and others. 



Magnetite is universally distributed. Apatite occurs in rare 

 abundance, size and perfection. Sphene and zircon are often 

 present, and epidote of somewhat exceptional character is very 

 common, especially in the mica-diorite from Stony Point. This 

 mineral is of a pale green color, without pleochroism and its 



p. 267). Prof. Dana objects to the term mica-diorite, because (1) the original 

 diorite was a hornblende rock and (2) because hornblende and mica are widely 

 different minerals. It must, however, be remembered that, although these two 

 minerals are co different, they play a very similar role in rock-composition. The 

 name mica-diorite is here retained because, in spite of all objections to it, it has 

 the very great advantage of being readily intelligible to students of petrography 

 the world over, — something that cannot be said of any other term which might be 

 proposed in its place. 



* This Journal, 1IT, xxxiii. p. 141, Feb., 1887. 



