Petrographical Relations of Laurmtian Limestones. 17 



distinctly banded, being made up of dark colored bands 

 alternating with bands of medium and light color. 



When examined microscopically, this banding is also 

 very apparent, as shown by the occurrence and arrange- 

 ment of certain minerals. It is thus rather difficult to 

 divide the constituents into essentials and accessories. 

 The dark bands are composed almost exclusively of 

 plagioclase, hornblende and augite. The lighter ones are 

 found to be interbanded in themselves, one which appears 

 homogeneous in ordinary light, may be seen, when the 

 second nicol is put in, to consist of bands possibly as 

 follows : — plagioclase, scapolite, and augite ; plagioclase, 

 microcline, calcite, and augite ; plagioclase, calcite and 

 augite. A few irregular individuals of quartz, numerous 

 fragments of sphene, many small crystalline grains of 

 apatite, a few grains of ilmenite, a little epidote, and a 

 very few irregular individuals of biotite make up the 

 accessory constituents. All the minerals, except the 

 calcite individuals, which are often of good size, are present 

 in small grains. 



The quartz, examined in isotropic section in convergent 

 polarized light, shows the stationary cross of a uniaxial 

 mineral ; by this means only is it distinguished from much 

 of the plagioclase, for considerable of the latter is clear 

 and un twinned. Some of the plagioclase, however, is 

 twinned, in broad and narrow bands. From the separa- 

 tion, there seems to be both labradorite and oligoclase 

 present. Microcline is recognized by its characteristic 

 twinning, and is also quite clear. A colorless mineral 

 whose optical properties show it to be scapolite occurs in 

 irregular individuals, at times cracked, and often turbid. 

 Its grains show sometimes parallel and sometimes rect- 

 angular cleavage lines ; its refractive index is low, but its 

 interference colors (double refraction) very brilliant. 

 Sections showing prismatic cleavage give parallel extinc- 

 tion, while those showing two sets of cleavage lines give 



