Petrographical Relations of Laurent ian Limestones 21 



iron ore, and here and there a small crystal of apatite 

 complete the list of accessory constituents. 



The quartz is clear, and is distinguished from much of 

 the plagioclase only by its axial figure. The orthoclase 

 is comparatively fresh, shows Carlsbad twinning in some 

 cases, also cleavage approaching a right angle, with 

 extinction sometimes parallel. Its occurrence in such 

 basic rocks is not infrequent. 1 The plagioclase is in good 

 sized individuals ; some of it, labradorite, is twinned 

 polysynthetically according to the Albite law, in broad 

 bands, but most of it is perfectly clear and untwinned, 

 and is andesine, specific gravity about 2.65. The scapolite 

 is usually altered about the border as in the other 

 specimen from this locality (section 1269), but otherwise 

 is quite clear. The hornblende is of medium green color, 

 and occurs in angular individuals of varying size. It 

 shows good cleavage, is strongly pleochroic, and exhibits 

 the usual absorption £>U>a, though in this case the 

 absorption along b and C is not very different. Its 

 extinction angle is 27°. The augite is in small grains, 

 much cracked. It is very pale green, almost non-pleo- 

 chroic, and gives an extinction angle of 40°. 



The rock appears as a mosaic of various sized grains* 

 less uniform than in the preceding specimens. Its 

 structure may be said to be between granular and granu- 

 litic, but still points to recrystallization of the component 

 minerals. 



INCLUSIONS IN THE GNESSIC GRANITES. 

 These comprise the irregular dark masses in the granite, 

 and are often near areas of gray gneiss. They present a 

 gradation in form, appearance, and composition from 

 angular, black, and exceedingly basic near the contact, to 

 lenticular masses, with alternately light and dark bands, 

 and less basic farther away in the granite mass. 



1. cf. Kemp, J. F., " Crystalline Limestones, Ophicalcites, and Associated Schists of 

 Eastern Adirondacks," Bull. Geol. Soc. Am., 1894, Vol. VI., p- 253. 



