346 GEOLOGY OF THE EUREKA DISTRICT. 



Appendix— Metamorphosed Sandstone— A micaceous fluc grained Tock occiiis ill sevcral 

 localities in the vicinity of Modoc Peak, which is traceable to thin beds of sandstone, 

 which, however, are never so full of mica, and though the true nature of its occurrence 

 is somewhat in doubt it is safe to consider it a highly altered forms of the same 

 quartzose deposit, since a series of thiu sections from the bedded sandstone and the 

 very micaceoas rock grades imperceptibly from one extreme to the other, the coarsest 

 grained variety having the mineral composition and structure of a microgranite. Of 

 the thin sections prepared, three, 437, 440, 4r>0, are from dense cryptocrystalline sand- 

 stone of a yellowish pink color, bearing a few quite perfect crystals of muscovite and 

 quartz. Under the microscope the rock is seen to be formed of minute quartz grains, 

 shreds of potash-mica and patches of a colorless cryptocrystalUue substance, through 

 all of which is scattered muchcalcite and ferrite, and occasionally' long, slender, beau- 

 tifully terminated crystals of zircon. The quartz grains range from 0-1 to 0-05""" in 

 diameter, and have the granitoid form, in no instance suggesting waterworn fragments. 

 Tliey contain extremely minute fluid inclusions, which literally swarm in the micro- 

 scopic quartz dihexahedrons of section 440. The form in which the calcite is found 

 suggests its alteration from feldspar, or its deposition by infiltration in the place of 

 decomposed feldspar, which is undoubtedly the case in one or two quite large sections. 

 Thin section 451 is very similar to those just mentioned, and with 452 came from a body 

 closely connected with the bed of sandstone represented by 450. The groundmass of 

 451 is the same in every respect as those just described, but there are numerous 

 macroscopical individuals of mica and feldspar, the latter still showing in some cases 

 the striping of twinned plagioclase, though mostly altered to a cryptocrystalline mass 

 like that occuiring in the gronndmass, which may probably have the same origin. 

 The poorly defined mica is completely replaced by calcite and ferrite. Sharply crys- 

 tallized zircon and apatite are present in stout prisms with very uneven outline. 

 The three remaining thin sections, 452, 466, and 463, exhibit the highest development 

 reached, and might be considered micaceous microgranite. The groundmass is com- 

 posed of granitoid quartz and partially altered feldspar in grains about O-l"" in diame- 

 ter together with shreds of potash-mica and calcite. In this lie porphyritically imbed- 

 ded well developed feldspar crystals and mica and occasionally quartz. The feldspar 

 is much altered, but shows that it is partly Carlsbad twins, and is partly striped 

 plagioclase. The mica is somewhat altered and is of brownish yellow color, with 

 strong absorption. A large individual of quartz in 466 has a multitude of minute 

 fluid inclusions arranged in planes parallel to the prism or rhombohedral faces. Ferrite, 

 apatite and zircon also occur. The whole is a thoroughly granite-like rock, without 

 signs of foliation. 



It is interesting to note in this connection how an apparent granitoid form of 

 quartz grains may sometimes arise from an entirely difierent cause. The phenomenon 

 is exhibited in a quartz conglomerate of small grain, thin section 501, where it is 



