MINERALOGY. 97 
This mineral shows very well the greater power which a pure, glassy 
feldspar possesses to resist decomposition, for it shows not the slightest 
change either in pieces or under the microscope. 
60. Oxicociasz [(Ca Na? K*) Al? Si® O"]. 
This, again, is an important and frequently occurring feldspar in our 
rocks. At Orange summit, specimens suitable for the cabinet are found, 
but its chief interest is lithological. The feldspars thus far considered 
are characteristic of basic rocks; but oligoclase being of a more acidic 
character, is often found with orthoclase in such rocks as granite, sienite, 
hornblende schist, and the like, and is more widely disseminated than 
was suspected before the microscope was brought to aid rock analysis. 
In thin sections of these rocks it is very easily distinguished from its 
associate orthoclase by its triclinic character, and its banded structure 
produced by twinning, which is so evident in polarized light. Some- 
times this mixture of orthoclase and oligoclase is macroscopically 
apparent, when the rock is coarse-grained in texture; for in such cases 
the feldspars are often of different shades of color, and the oligoclase 
can be distinguished by its greater tendency to alteration, which 
causes cleavage surfaces, where exposed, to appear duller than do those 
of the orthoclase; and though cases exactly the reverse have been 
observed by Zirkel and Rosenbusch, it may be regarded as the general 
rule. It may also be distinguished by the striations, which are some- 
times apparent, and which are superficially developed, sometimes by 
weathering, when they cannot be seen on fresh surfaces of rock. Their 
presence may be regarded as conclusive of the triclinic character of the 
feldspar, though their absence is not equally so. 
In sections of the rock the bands of color produced by a twinning 
are often extremely narrow, and the absence of superficial striation on 
oligoclase is perhaps often due to the extreme fineness of the lines. 
In Fig. 6 on Pl. 7, a basal section of a grain of oligoclase from the 
Antrim granite is shown. The bands of color are very thin, there being 
here a thousand to an inch, and they are also of extreme regularity, as is 
quite usual in the oligoclase of granites. The section is so placed in the 
drawing as that one set of laminze are dark, and the lamin make an 
angle of 3° with the spider line in the ocular. It must be turned 6° in 
VOL. IV. 13 
