272 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 39 



the eastern edge of this flood plain, and the extension of the cliff 

 to the south show the downward extension and hard rock phase of 

 the weathered outcrops described later as exposed along Adams 

 Mill Road (fig. 1, A, and pi. 8). The bear pits were built in the 

 granite by the removal of a few of the rectangular joint blocks 

 formed by twisting of the earth's crust, the cavities thus affording 

 an idea of their size, shape, and position. The hard granite here 

 is cut by thin veins of quartz sometimes crisscrossing each other. 

 Metamorphism through pressure and heat has changed some of 

 the layers into gneiss with its minerals compressed into thin parallel 

 bands. At the top of the outcrop, Aveathering from the surface 

 waters becomes more evident, and with the removal of the soluble 

 feldspar the quartz gi^ains and mica flakes become loosened, al- 

 though the rock still retains its jointed structure. At the very* 

 top of this outcrop along the edge of the Zoo on Adams Mill 

 Eoad the disintegration has become complete with the formation 

 of a residual soil of sand and mica flakes mixed with present-day 

 vegetable matter. On either side of this granitic igneous intrusion 

 the rock formation changes to highly foliated, hard layers of dark- 

 colored mica schist speckled with iron pyrite crystals and sparkling 

 in the sun upon fresh fracture. The scMstosity of the rock, due to 

 the parallel arrangement of the particles because of the enormous 

 pressure it has undergone, and the bedding planes of the original 

 sediments possibly coincide in this area ; at any rate, the nrica schist 

 outcrops here have the aspect of an upfold or anticline overturned 

 to the northwest. Mica schist outcrops continue for some distance 

 to the south (fig. 1, C)^ first as highly tilted strata representing a 

 compressed fold, and then with less inclined strata in the form of 

 a low downfold or syncline. Here, as in the granite, fractures in 

 the rock of various length and thickness have been filled with vein 

 quartz which weathers so slowly that it stands out above the general 

 surface; some of these are auriferous quartz veins, as rarely small 

 particles of gold can be seen intertwined with the quartz. Through 

 weathering in the course of the ages such veins have contributed 

 enough gold to the stream sands to make placer tnining possible if 

 not profitable, as a slight color of gold can be obtained by panning 

 the gravels of certain streams in this area. Elsewhere in the park 

 these quartz veins sometimes attain considerable thickness and be- 

 cause of their slow disintegration remain well-defined hills. At 

 still other places where the schist comes in contact with either 

 granite or gneiss, minerals are developed through contact meta- 

 morphism,, garnets being found usually in such circumstances. 

 Again, here and there former deep fractures in the earth's crust 

 have been filled by the intrusion of magma, deeply buried, molten 



