282 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



Granite gneiss, District of Columbia; (2) granite, Stone Mountain, 

 Georgia; (3) nephelin syenites, Arkansas; (4) diorite from Vir- 

 ginia; (5) diabase from Massachusetts; (6) dolomite from South 

 Dover, N. Y. ; (7) limestone from Maryland and Virginia; and 

 (8) slate from Maryland. Attention may be called to the fer- 

 ruginous clay representing the ultimate products of decomposition 

 of rocks of nearly every type regardless of original composition. 

 A series of typical examples of the East Indian laterites is of in- 

 terest in this connection. A series of pegmatites from Blanford, 

 Mass., well illustrates the origin of kaolin through feldspathic de- 

 composition. Characteristic boulders of exfoliation are shown in 

 diabase from Massachusetts and Virginia. A strikingly peculiar 

 form of weathering is displayed by a large block of siliceous 

 conglomerate from Wise County, Va. The block consists of water- 

 worn pebbles of white quartz embedded in a finer ground of com- 

 pact siliceous sand. The block was taken from the immediate 

 surface and wherever the quartz pebbles projected so as to expose 

 them to the weather, it will be observed they are more or less cor- 

 roded and in some cases entirely eaten away. Where the pebbles 

 lie beneath the surface they have escaped corrosion. The cause of the 

 phenomena is not apparent, but it is probably both physical and 

 chemical in its nature. 



Examples of unequal and uneven weathering due to inequalities 

 in composition are shown in the schist traversed by small quartz 

 veins from Rhode Island. The quartz being most refractory stands 

 out in relief as the softer portions weather away. The eroding 

 action of wind-blown sand is shown in beautifully polished "sand 

 blasted" masses of quartzite from Minnesota, in the irregularly 

 and sometimes grotesquely carved sandstone from various points 

 in the West, the carved pebbles ("dreikante") from several sea 

 beaches, and a window pane from a lighthouse at Nauset, Mass., 

 etched by wind-blown sand during a storm of but a few hours. 



Illustrative of special phases of weathering, attention may be 

 called to the thin sheets of granite and porphyry flaked off from the 

 main ledges through expansion and contraction caused by natural 

 changes of temperature. The more striking examples are from 

 Ellsmere Land and Mount Kineo, Me. 



A large block of sandstone from Arkansas shows induration along 

 joint planes due to a segregation of iron oxide, the oxidation being 

 brought about by the percolation of water along the joints. The 

 intervening material being robbed of its cementing constituent crum- 

 bles away under the influence of the weather, leaving the hollows. 



