274 AiTNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 9 



jasper^ rose quartz^ yellow {citrine) quartz, and chalcedony from 

 veins in the igneous rocks, and several types of the more insoluble 

 rocks of the Piedmont Plateau among these afford a field for the 

 mineral collector. Boulders of the sedimentary formations are 

 represented by quartzite with Scolithus worm tubes from the Lower 

 Cambrian of the Blue Ridge, iron-stained sandstone of the Silurian 

 Clinton strata of the Appalachian Valley, saccharoidal sandstone 

 from which glass is now made, and -flints and cherts with character- 

 istic Devonian fossils (trilobites, corals, etc.), from the Allegheny 

 Mountains. Limestones and other soluble rocks, even including the 

 granites and gneisses, are absent because, although originally present, 

 they have been dissolved away long ago. The water thus has another 

 erosional power through its ability to dissolve rocks by the process 

 called corrosion. An example is seen in the joint block of granite on 

 the flood plain previously described (fig. 1, F, and pi. 4, fig. 1), 

 dragged here in 1902, which has weathered to such an extent in the 

 intervening time that the edges are rounded and the whole surface is 

 pitted with spaces once occupied with feldspar crystals. A narrow 

 quartz vein, now standing out conspicuously on account of its slight 

 solubility, exhibits the process of differential weathering. 



The many granite outcrops in the Park, exhibit other important 

 phases of weathering of the hard rocks into soils. Along Tilden 

 Street leading down to Pierce's Mill (pi. 5, fig. 2) is an old quarry 

 exposure which lends itself to photography better than those in the 

 Zoo. Here within a few feet the change from the solid granite with 

 jointed structure into the decayed subsoil above still retaining the 

 outlines of the joint planes., and, finally, the soil with present-day 

 plant growth, is quite evident. In this process of weathering by solu- 

 tion, boulders are sometimes formed which, when carried away by 

 streams, may be deposited with boulders of other origin to form the 

 sedimentary rock, conglomerate. The formation of such residual 

 hotdders (pi. 5, fig. 1) occurs when water percolating, for example, 

 along the joint planes of the granite, slowly disintegrates by solution 

 the outer parts of the blocks into decayed material, leaving the central 

 part the last to be attacked. This then, unless the waters have had 

 time to permeate to the very center and cause the decay of the entire 

 block, remains as a hard rounded mass to be washed out as a boulder 

 at some later time. This process results in spherical boulders, whence 

 the name spheroidal weathering. Since water is a necessity in the 

 process, such weathering is an indication of a moist climate. In 

 dryer climates or where there is considerable alternation of heat and 

 cold, as in the very hot days and cold nights of desert regions, 

 exfoliation., a special type of weathering, occurs. Here the outer 

 layers of such a boulder expand under the action of heat but upon 



