62 GEOLOGY OF TPIE COMSTOCK LODE. 



be visible in the narrow lamellae as well as in the broad ones. Tlie intrn- 

 sive groundmass, too, is scarcely compatible with the supposition of variable 

 tension, and the zonal structure in this case must be due to modifications 

 in chemical composition. This may vary in two ways; there may be a sub- 

 stitution of isomorphous elements without disturbance of the characteristic 

 "oxygen ratio" (atomic ratio) of the mineral species, or this ratio may be 

 modified in the sense of Professor Tschermak's feldspar theory. Granting 

 the accuracy, or even the approximate accuracy, of Messrs. Fouque & 

 Lcivy's discussion of the optical properties of labradorite' and other feld- 

 spars, the first supposition is impossible in the present case; for if, at tlie 

 position indicated by the angle of extinction of the thin lamella; and two of 

 the zones, this angle may vary 10°, the distinction of different species by 

 this property is illusory. Indeed the extinctions are consistent with the sup- 

 position that the intermediate belt is oligoclase, an hypothesis, however, 

 with which the crystallographic unit}' of the section is incompatible. I am 

 therefore forced to the supposition that the intermediate belt answers to a 

 variety of feldspar of a diflFerent oxygen ratio from labradorite, but crystal- 

 lizing in this mixture in the same form." The same explanation seems to 

 me indicated in most zonally-built plagioclases, and in those which display 

 progressive divergence of the optical axes. 



AUGITE-ANDESITE. 



General character. — TliG augite-andesitcs prcscut the closest parallellism to 

 thehornblende-andesites; the resemblance being far closer than that which 

 exists, for example, between the diorite and the diabase. But for the fact 

 that they clearly belong to diffei'ent eruptions it would seem more appropriate 

 to regard the two rocks as varieties rather than as independent species. In 

 the Washoe district the porphyritic augites are rarely macroscopically 

 noticeable, but their effect is perceptible in a certain resinous luster. While 

 the color of the earlier hornblende-andesite in a fresh condition is commonly 



'L. c, p. 253 



°The influeuce of salt.s of analogous properties, when mixed, iu modifying the resultant crystal 

 form is well-known. 



