284 ANNUAL REPORT SMITHSONIAN INSTITUTION. 1921. 



THE MECHANICAL ACTION OF WAVES IN BREAKING DOWN ROCKY CLIFFS 



AND THE FORMATION OF PEBBLES. 



The exhibit begins with characteristic slabs of a schist with in- 

 cluded quartz veins from the coast of Cape Elizabeth, Me. The 

 continual hammering of the waves causes the schist to slowly dis- 

 integrate and fall to the foot of the cliff. The quartz veins being 

 the hardest and toughest remain intact to the last, as shown in one 

 of the larger specimens (36036). Once at the foot of the cliff, the 

 fragments are alternately thrown upon the beach and dragged back 

 into the sea by each successive wave and its return undertow, until 

 gradually reduced to the pebble form. All stages in the process 

 are shown, from the angular fragment as it fell from the cliff to 

 the resultant oval pebbles. It will be observed that owing to the 

 fissile nature of the schist (70056) its pebbles are always in the 

 form of a greatly flattened oval, while those of the quartz (70055) 

 are more nearly spherical. At the end of the series is a coarse sand 

 composed of fragments of schist, quartz, and shells, and which was 

 obtained at low tide further out from the shore. 



This exhibit is accompanied by one designed to show the manner 

 in which sedimentary materials are derived through the breaking 

 down of rock masses through the ordinary processes of weathering. 

 For this purpose the granitic rock of Rock Creek Valley in Washing- 

 ton has been selected. The exhibit begins with (1) a sample of fresh 

 granite, (2) one of partially decomposed granite, and (3) a jar of 

 the material decomposed to the condition of a coarse granitic soil. 

 The eight succeeding jars contain an amount of soil equal to that in 

 the large jar, which has been separated into portions of fairly uni- 

 form sizes by sifting and washing. The individual labels give the 

 percentage amounts of the various portions as compared with that 

 in the larger jar. 



Attention is called to the fact that, by the consolidation of such 

 sediments, there might be formed a series of clastic rocks ranging 

 from coarse to fine, which would be classified as conglomerates and 

 breccias, coarse and fine sandstones, shales and argillites. 



FAULTS, FOLDS, ETC. 



Effects of earth stresses. — Rock masses subjected to great compres- 

 sive force or tension are sometimes distorted, crushed, or faulted in 

 a very striking manner and on $, scale such as can not be shown other 

 than by photographs or other form of illustration. Frequently, 

 however, blocks of a size suitable to museum exhibition are found. 

 Such are exhibited here. Attention need be called to the folded 

 jaspery hematite, (Spec. No. 70266), from Ishpeming, Mich., and 



