STAGES IN THE METAMORPHISM OF MICA-SLATES. 223 



belt, the rocks become more crystalline and grade into the schists about the 

 volcanics to the north and the granite of Harney peak to the south. In the 

 field no unmistakable fragments have been found by me in the schists 

 immediately adjacent either to the granite or the volcanics, although certain 

 obscure forms were seen which may represent what may have been fragments. 

 In the transition in both directions, greywacke-slates change to mica slates ; 

 the mica-slates to non-foliated mica-schists ; the non-foliated mica-schists into 

 foliated mica-schists (which are both garnetiferous and staurolitic), and even 

 into gneisses. This gradation is not made out in any one continuous ex- 

 posure, but by many sections of detached exposures, in all of which the same 

 phenomena are observed. 



The steps in the process of transformation, as seen under the microscope, 

 are in many respects like those I have already described as occurring in the 

 upper slates of the Penokee series.* Later, Bonney f described some mica- 

 slates which have a similar origin. In the Black Hills, however, the result- 

 ing crystalline schists are coarser grained and more foliated than any of 

 these rocks. Also, unlike those of the Penokee area, they have been sub- 

 jected to powerful dynamic action, and this has had an important influence 

 in their development. The processes, in brief, which have changed these 

 once detrital quartz-feldspar rocks to thoroughly crystalline mica-schists are, 

 first, the alteration of the feldspar to the minerals muscovite, biotite and 

 quartz ; and, second, the breaking down of the larger clastic quartz individ- 

 uals by mechanical action. The first of these processes I have already 

 described in detail in the paper alluded to. By it crystalline schists are pro- 

 duced from feldspar detritus. These details I need not repeat ; but the de- 

 composition of fragmental feldspar is most beautifully shown in the Black 

 Hills rocks. It will suffice to say that as a result of this process an intri- 

 cately interlocking mass of crystalline quartz, feldspar and mica, or quartz 

 and mica, are produced from each of the large grains of clastic feldspar (figs. 

 1 and 2, plate 4). Usually many independent individuals of quartz and 

 mica occupy the space once taken by a single individual of feldspar. The 

 reticulating residual feldspar for a given fragmental grain acts as a unit, 

 except the process of recrystallization results in the formation of feldspar 

 of a different kind from the allothigenic individual. When the process is 

 complete, the interlocking mass consists wholly of quartz and mica. This 

 alteration is chemically possible because the micas, both biotite and musco- 

 vite, are much more basic than feldspar and the residual silica separates as 

 quartz. By imperceptible steps all phases of the alteration are seen, from 



* Upon the Origin of the Mica-Schists and Black Mica-Slates of the Penokee-Gogebic Iron-Bear- 

 ing Series, C. R. Van Hise: Am. Jour. Sci., :'.<! ser., Vol. XXXI, 1886, pp, 453-459. 



i i in some Results of Pressure and of the Intrusion ot Granite in Stratified Paleozoic Rocks near 

 Morlaix, in Brittany; < >u the Obermittweida Conglomerate, its Composition and Alteration : Notes 

 on a Part of the Hu'ronian ^erie* in the Neighborhood of Sudbury (Canada), by T. G. Bonney : Quart. 

 Jour. Geol. Soc., London, Vol. XLIV, 1888, Part I, pp. 11-19, 25-31, 32-44. 



