MINERALOGY. 51 



and then, between crossed Nicols, it will not show colors, but will approach 

 towards the behavior of other hexagonal minerals. If the section be a 

 little thicker, — of the thickness of ordinary rock sections, — the light will 

 not be rotated to such a degree as to separate the prismatic colors so 

 widely from one another as that one can see that the mineral exercises 

 circular polarization ; but still the section will not be entirely dark be- 

 tween crossed Nicols, and will not become so on rotating it in its plane. 

 Sections cut at all varying from the basal plane polarize the light, giving 

 the most brilliant interference colors. 



It may therefore be said that, in microscopic sections of rocks that are 

 of the proper thinness, quartz, although optically peculiar, does not differ 

 essentially from ordinary hexagonal minerals, and that its colors in polar- 

 ized light are peculiarly brilliant. Other microscopic characters are the 

 uniformity of these interference colors over its whole surface, except at 

 the edge of its crystals or grains. The edges are differently colored, on 

 account of the varying thickness at these points. The almost uniform 

 presence in it of little cavities filled with fluid, and containing bubbles 

 and often little crystals, is noticeable. These cavities are often hex- 

 agonal. Figures of them will be found in the plates. 



As quartz was, as a rule, the last mineral to crystallize in our rocks, it 

 is more often in rounded or irregular grains than almost any other min- 

 eral. As it is very difficult to decompose, it is ordinarily clear and trans- 

 parent in thin sections, while the minerals that surround it are more or 

 less decomposed. It possesses no cleavage, and when in large particles 

 is usually traversed by irregular fractures. In a fine-grained mixture of 

 quartz and orthoclase, the two minerals are not easily distinguished from 

 one another, as they both give brilliant colors in polarized light, and the 

 cleavage and other properties of the feldspar cannot be recognized. In 

 such cases, in examining sections of our old rocks, the presence of the 

 two minerals together is best recognized in the microscope by shutting 

 off the light from below, and examining by reflected light, when the 

 quartz, which is clear and undecomposed, appears black, while the feld- 

 spar usually appears as a white, opaque, snowy substance, this effect 

 being produced by its impurities, minute fissures, and partial decomposi- 

 tion. 



