R CK-S TR UCT URE. 



429 



feldspar, somewhat opaque and cloudy as they usually are in granite, 

 but now and then clear and beautifully striped, and also the crystals 

 of mica, imbedded in the clear quartz, which will be at once known 

 by its bright clear colors and by the margin of rainbow-like tints 

 which border its patches. Ordinary orthoclase feldspar is usually some- 

 what opaque and dirty-looking under the microscope, and by this it 

 may be distinguished from the clear, glassy sanidine which is fre- 

 quently found in igneous rocks, and presents under the microscope, 

 when polarized, pure rich colors as well as sharply-defined crystals 

 similar in form to those of the common orthoclase. The orthoclastic 

 feldspars may be very readily distinguished from the plagioclastic by 

 their structure, as revealed by the polariscope ; the latter invariably 

 are seen to be striped with variously-colored bands, showing what is 

 called twin crystallization ; and the orthoclase, though often forming 

 twins on a larger scale, does not present the minutely-banded appear- 



Fig. 4. Mica (Biotite). 



ance of the plagioclastic feldspars. The mica in the granite section 

 will not be difficult to recognize, especially if Biotite ; often we shall 

 observe it as forming fairly-shaped hexagonal crystals, and the polar- 

 iscope will also help us to know it by its thinly-laminated structure, 

 giving rise to fine parallel strise on the surface of its crystals. Its 

 colors, also, when polarized will be duller than those of the quartz, 

 for which it might sometimes be mistaken at first sight, should it be a 

 light-colored mica ; and then, again, it will frequently be found that 

 when the prisms of the polariscope are crossed the mica becomes per- 

 fectly opaque, its sections having been formed across the optical axis. 

 But let us now look at the quartz. We shall observe that this quartz 

 is generally not crystallized in definite forms, as are the feldspar and 

 the mica ; it appears as a matrix which has been at some time or other 

 soft and so is penetrated by the other crystals, the interspaces of 

 which it fills up : this shows us at once that it must have been solidi- 

 fied after them, and so was unable to assume its regular forms. This 

 is a very remarkable fact, and helps us toward the secret of the forma- 

 tion of the granite. We know that quartz requires a higher tempera- 

 ture to melt it than does either the feldspar or the mica, and so, had 

 the granite been formed as are regular volcanic rocks in the ordinary 

 way of igneous fusion, we should certainly have found that the quartz 

 would have crystallized before either the feldspar or the mica, and it 



