STAGES IN THE METAMOE.PHISM OF QUARTZITE. 210 



ever, does not appear. In no ease have I been able to find a truly granitic 

 pebble in the conglomerates, although the presence of feldspar, both ortho- 

 clase and plagioclase, in the quartzites aud conglomerates indicates that they 

 have probably been derived from some such rock as gneiss or granite. The 

 identification of the source of detritus is in general a very difficult thing to 

 do, and that in this case the material was from the particular granite and 

 schists now exposed in the Black Hills seems to have been assumed without 

 proof. 



The feldspar of the quartzites and conglomerates has usually decomposed 

 to such an extent as to have lost its original rounded character. The re- 

 sultant products are muscovite, biotite, and less frequently kaolin, accom- 

 panied by a simultaneous separation of quartz. Generally the decompo- 

 sition has taken place to the greatest extent upon the exterior of the grains, 

 but affects them, more or less, quite to their interiors. In some sections all 

 stages of the change are seen, from that in which the mica forms a circle of 

 folia about and penetrating a feldspathic grain to that in which nothing 

 remains of it. 



The interstitial material in the quartzites and conglomerates is chiefly 

 finely crystalline quartz which has been deposited as independent particles. 

 The induration of the rocks is then due both to the enlargement of the old 

 grains and to the deposition of new quartz. Pressure also may have had its 

 influence. The total amount of infiltrated silica is very considerable, although 

 the fragmental grains are of various sizes, fit closely, and consequently leave 

 an unusually small amount of interstitial space. This amount of deposited 

 quartz is increased by numerous quartz veins. 



The fact that iron oxide has oftentimes been a subordinate filling material 

 makes it frequently easy to determine just what part of the quartz is an 

 original detrital material and what a secondary deposition, the former ex- 

 cluding and the latter including the ferrite. Accompanying the interstitial 

 quartz and iron oxide is a greater or less quantity of muscovite or sericite. 

 or both. The quantity becomes so great in certain cases that the rocks could 

 well be called a muscovitic or sericitic quartzite, while it occasionally passes 

 over into a muscovite-slate, or sericite-slate, or schist of the same kind. Other 

 minerals, such as iron and other carbonates, and tourmaline, are present as 

 infrequent additional accessories. 



The micaceous slates associated with the quartzites and conglomerates 

 differ from them only in that the amount of feldspar in the original detritus 

 has been greater and the particles of smaller size. The decomposition of 

 this mineral has produced both biotite aud muscovite abundantly and the 

 luck has passed over into a slate. The nature of this process will appear 

 later in more detail. 



The quartzites and conglomerates above described differ profoundly from 



XXIX— Bull. Geol. Sue. Am.. Vol. l, 1889. 



