1894. PROCEEDINGS OF THE NATION'AL MUSEUM. 653 



the predominating constituent. Slireds of brown mica, abundant gran- 

 ules of iron ore, occasionally a little interstitial quartz, and a few 

 inconspicious apatites with rarely an irregular sphene complete the 

 list of determinable constituents. Others differ mainly in being of 

 coarser texture and showing a tendency toward aporphyritic structure 

 through the development of occasional large plagioclases. 



Two sections show pecnliar modifications. The most abundant con- 

 stituent is a pale green augite which has gone over to a considerable 

 extent to chlorite and a uralitic hornblende. Both augites and plagio- 

 clases show a tendency to group themselves into granular aggregates, 

 while the latter are in many instances so cliarged with globular, club- 

 shaped or vermicular colorless inclosures as to almost obscure their 

 true mineral nature. 



The feldspar not infrequently appears as a ragged, irregular nucleal 

 area, with or without twin striae, and surrounded by a zone of varying 

 width of the inclusions so as to appear, by a low power, like a very 

 fine granular aggregate of colorless minerals. A single section shows 

 all gradations from feldspars with no inclusions to areas no longer 

 recognizable as such, but merely aggregates of the irregular inclusions 

 described. The structure is at times pronouncedly cataclastic and 

 the appearances such as to indicate that the above-mentioned peculiar- 

 ities are due to dynamic causes. 



Rhyoliteand andesite. — Hills between North and South Meadow Creek, 

 near Washington Bar. The eruptives here occur mainly on the eastern 

 side of the creek and form the steep hills lying between the north and 

 south branches. The predominant rock is a liparite overlying the 

 gneiss and forming the main mass of the bill. This varies from pur- 

 plish to gray or nearly white in color, sometimes pinkish. The ordinary 

 type is faintly porphyritic, though sometimes quite aphanitic, and with 

 so even and pronounced a flow structure as to closely simulate a com- 

 pact, thin bedded, argillaceous limestone. On the eastern and nj^per 

 slope of the hills, andparticularly at the western end of tlie range, always 

 near the contact between the liparite and gneiss there occur limited 

 outcrops of a dense almost coal black andesite. (No. 72850, U.S.N.M). 

 The contacts between the three rocks are never accessible, but the 

 surface of the ground is simply covered with innumerable small joint 

 blocks, rarely a foot in diameter, and which have been further reduced 

 by the continual flaking oft' of small convex and concave chips by 

 diurnal temperature variations. The rock on a weathered surface for 

 perhaps the depth of a millimeter is of a brownish color. Beyond this 

 it is always fresh, of a beautiful dense black color and satiny luster, 

 and with only rarely small white i^orphyritic constituents suflQciently 

 developed to be visible to the unaided eye. 



A slight mottling souietimes seen on the surface is due to a spheru- 

 litic development. The field characteristic of the rock was such that 



