62 QUICKSILVER DEPOSITS OP THE PACIFIC SLOPE. 



acteristic cleavages, extinctions, and twinning. Oligoclase is the most 

 abundant triclinic feldspar, but in a few cases angles of extinction between 

 20° and 30^ on each side of the twinning j^lane indicate the presence of 

 more basic species. Biotite is also a constituent frequent in many of the 

 sandstones. When it occurs it is usually allothigenetic, and this is shown 

 by its relation to tlie structure of the mass, the scales being distorted by 

 the pressure of the unyielding grains of quartz and feldspar about it. Occa- 

 sionally biotites appear to have been bruised edgewise and very finely di- 

 vided clastic material has silted in between the contorted foils. The biotite 

 wlien fresh is dirty brown in color and in no respect differs from that of the 

 granites. That a white mica, probably muscovite, forms in the sandstones 

 epigeneticallv from biotite is certain. It is also found in such a way as 

 to suggest that it is allothigenetic, but it is not impossible to explain these 

 cases by epigenesis. While it is altogether probable on general principles 

 that the muscovite of the granite is represented in the sandstone, the nature 

 of the case precludes absolute certainty on this point. Muscovite is a very 

 subordinate constituent of the granite. 



Hornblende, exactly like the granitic hornblende, is tolerably common 

 ia the sandstones, usually in very small grains. Titanite in rounded grains, 

 minute zircons, and occasionally epidote have been observed. A strongly 

 refracting, monochroitic mineral was detected, which, after separation, was 

 proved by chemical tests to be rutile. Tourmaline in large, brown, in- 

 tensely dichroitic grains was also found in the same sandstone as the rutile. 

 Small apatites, especially included in the clastic quartz grains, are not un- 

 common in the sandstones. Some of the slides of granite in the collection 

 show more apatite in the quartz grains than do any of the slides of sand- 

 stone, but apatite is rather irregularly distributed in the granite and some 

 thin sections of this rock contain extremely little of it. 



The only allothigenetic material not derivable from the granite which 

 appears with any considerable frequency consists of occasional black scales, 

 sparsely distributed in some localities, from the Knoxville group upwards. 

 In many cases these scales seem referable to carbonaceous shale; in others 

 they at least suggest plant remains, such as are found in the schists of the 

 Knoxville series. 



