140 G. H. Williams—WNorites of the “ Cortlandt Series.” 
degree the peculiar “ shagreen ” surface, so characteristic of cer- 
1. tain minerals such as olivine (fig. 1). 
This appears to be due to a vast num- 
ber of very minute oval indentations 
which completely cover the surface 
wherever it is exposed. 
This occurrence of orthoclase in 
norite is very interesting and is quite. 
in: accord with determinations for- 
merly made of the feldspars in these 
rocks by Prof. Dana and the late Dr. 
G Wi Glawes:* 
The plagioclase, which composes 
most of the groundmass of this rock and also occurs oceasion- 
ally as inclusions in the orthoclase, is shown by its specific 
gravity and extinction angles to be Andesine. The former, as 
determined by the Thoulet solution, is 2°674. The extinction 
angles are 4°-5° on OP (001) and 10°-12° onw P& (010). In 
the groundmass of the rock these plagioclase individuals are 
crowded thickly together and are often bent or broken. In 
such cases they show most beautifully the increase in the num- 
ber of twinning lamellze where the strain has been greatest (fig. 2.) 
2 Such a mechanical origin of twin- 
ning lamelle in feldspar has been 
figured and described by van Wer- 
vecke+ and Judd,t but it would 
be difficult to conceive of finer ex- 
amples of it than are to be found in 
this rock. In the groundmass these 
plagioclase crystals are well devel- 
oped and have their characteristic 
lath-shaped form, but where they 
are imbedded in the orthoclase they 
show exceedingly irregular shapes as 
Section No. 43. Bent and though they had been partially dis- 
broken plagioclase crystals eae (Gemeal 
showing secondary twinning SUS ( o ). : 
lamellz. On the leftand near Lhe hypersthene is generally ar- 
es ee eee Onthe ranged in groups of three or more 
right orthoclase enclosing an .., 74, aa 
aeoalae Maplaeraclasolmictertall crystals. These are intensely colored 
Plagioclase is represented be- OWing to the large amount of iron 
en crossed Nicols in order to which they contain and show their 
pe out its twinning aerate characteristic trichroism (a = 7 =red; 
6=h= yellow: ¢=¢=green) very vividly. Inspite of their 
deep color, these hypersthene crystals are unusually full of the 
* This Journal, Sept. 1880, p. 197. 
+ Neues Jahrbuch fiir Mineralogie, etc., 1883, ii, p. 97. 
¢ Quarterly Journal of the Geological Society, Aug. 1885, p. 365. 
