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. Oligoci.ase [(Ca Na'^ K^) Al- Si^' 0"]. 



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 PI. 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 oHgoclase of granites. The section is so placed in the 

 drawing as that one set of laminae are dark, and the laminas make an 

 angle of 3° with the spider line in the ocular. It must be turned 6° in 

 VOL. IV. 13 



