284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1938 



distances from their source. A little further consideration may now 

 be given to the effect of volcanic action. 



Our attempts to penetrate the superficial layer of the ocean floor 

 are necessarily very limited, and the likelihood of establishing the 

 nature of the foundation rock of the crust is remote. But indirect 

 evidence is afforded by the submarine volcanoes which are numerous 

 and widespread in the oceanic areas. Many are of immense size and 

 some rise, like Mauna Loa in the Hawaiian Islands, from the ocean 

 depths to heights of 13,600 feet above sea level. Indeed, most of the 

 islands which he far from continental shores are composed exclusively of 

 rocks which have cooled from the molten state, and mostly of rocks of 

 volcanic origin as distinct from deep-seated origin, i. e., those that have 

 solidified in the depths of the earth's crust. The rock basalt, familiar 

 to all in the Giant's Causeway and the Hebrides, is of commonest 

 occurrence in oceanic islands, and the broad similarity of the chemical 

 composition of specimens from widely separated island localities 

 points to derivation from a common subcrustal region or shell. In 

 some islands there is a sorting-out of mineral (i. e., chemical) con- 

 stituents, known as differentiation, which leads to the formation by 

 crystallization of various rock-types, but these are rarities from the 

 standpoint of total bulk. 



It is common knowledge that when lavas are emitted from vol- 

 canoes on dry land, they not infrequently (like Vesuvius, for example) 

 float up fragments of the "country-rock" more or less baked and some- 

 times in part assimilated at their margins. The "intrusive" representa- 

 tives of the lava, that is, the feeders which become consolidated before 

 they reach the surface, contain many more such "xenoliths." In a 

 country where the crustal rocks are concealed by the volcanic pile, 

 useful information is thus afforded of the nature of the superficial 

 crust. But in oceanic areas such xenoliths of continental rocks are 

 very scarce or not found in the lavas, the records that exist being 

 nonproven. Hence has arisen the view that the foundation of the 

 ocean is the same as that below the continental masses (or sial), 

 namely, the subcrustal layer of approximately basaltic composition 

 known as the sima. Support for this deduction is afforded by the rate 

 of propagation of earthquake waves. 



Volcanoes on land, or below the sea in maritime areas, are frequently 

 of explosive type, the rocks being blown to fragments, even to dust, 

 and the products distributed over a relatively wide area. The 

 fragmental materials there consist of broken-up crystalline or glassy 

 lava or disintegrated continental rocks. In the case of submarine 

 volcanoes it is reasonable to suppose that if explosive action occurs 

 (although it may be damped down by the great load of the ocean), 

 the products would be distributed throughout the deep-sea sediments 

 at present being laid down. Now, examination of large numbers of 



