876 A TREATISE ON METAMORPHISM. 



profound, a schistose structure is strongly developed. Where the original 

 sands were of the same general character, schists are likely to be produced ; 

 where the sands were in alternate bands, some of which were mainly 

 quartzose and others strongly feldspathic, a gneiss is likely to form. 

 Such a gneiss is beautifully illustrated in the Sturgeon River tongue of the 

 Crystal Falls district of .Michigan, adjacent to the conglomerate-gneiss 

 already mentioned. (See p. 860.) 



During the transformation the quartz is granulated and recrystallized, 

 precisely as in the schistose quartzites. Since the feldspars of the arkoses 

 dominantly belong to the acid end of the series, they usually break up into 

 mica aud quartz. So far as the feldspar is orthoclase or microcline, musco- 

 vite and quartz form. Where the arkose is impure, and magnesium and 

 iron are present in considerable quantity, biotite is likely to develop. At 

 the stage i'i winch the alteration of the feldspar is partial there is an inter- 

 locking fixture of quartz, feldspar, and mica. In other words, at this 

 stage the rocks have a composition which, under the German usage of the 

 ierm, would be called mica-gneiss. Where among' the feldspars interme 

 diate varieties, such as labradorite and bytowuite, are present, simultane- 

 ously with the development of the other minerals, albite, oligoclase, and 

 andesine may form, but of these albite is by far the most prevalent. Thus 

 out of the old feldspar a great deal of new and usually acid feldspar is 

 produced. The development of new feldspar is particularly likely to take 

 place when adjacent igneous intrusions have occurred, especially granitic 

 intrusions. Doubtless the hot mass of granite promotes the formation of 

 solutions which carry abundant feldspathic material, and out of this, in part 

 at least, the new feldspar develops. (See pp. 713-715.) In the schist-arkoses 

 the micas have well-developed parallel orientation, with their longer axes 

 and cleavages in a common plane. The new feldspars may also have a 

 similar orientation. Such an arrangement is beautifully illustrated in the 

 Hoosac schist, described by Wolff. 6 In this rock, mainly composed of 

 quartz, mica, and feldspar, the latter mineral, mainly albite, is wholly 

 recrystallized and locally shows a very marked parallel orientation. 



« Clements, J. Morgan, and Smyth, H. L., with Bayley, W. S., and Van Hise, C. R., The Crystal 

 Falls iron-bearing district of Michigan: Mon. O. S. Geol. Survey, vol. 36, 1899, pp. 463-464. 



sPumpelly, Raphael, Wolff, J. E., and Dale, T. Nelson, Geology of the Green Mountains in 

 Massachusetts: Mon. IT. S. Geol. Survey, vol. 23, 1894, pp. 59-63. 



