SUBMARINE MOUNTAINS 49 



most abundant among all the types of lava, whether on land 

 or at sea. 



A host of other islands are in contrast. They are neither 

 basaltic nor essentially of volcanic origin at all; they are made 

 of rock species that are staple only in the continents and large 

 islands. The "continental" species carry high proportions of 

 the compounds called silica and alumina. From these names, 

 as we learned in the first chapter, the mnemonic word "sial" 

 has been coined, to cover "continental" rocks in general. 

 Islands composed of the sial we shall describe as "sialic." The 

 basalt of the volcanic islands, being rich in silica and magnesia, 

 may in a corresponding way be described as "simatic." 



Let us search the two classes of islands, in succession, for 

 clues concerning the properties of the earth-shells beneath the 

 wide ocean. 



The clues already discovered would have been better and 

 more numerous if the most economical program for wresting 

 the secrets of the oceanic islands had been adopted. This in- 

 volves prolonged co-operation of specialists, covering group 

 after group and thereby gaining in mastery of the island tech- 

 nique. The geologist needs detailed information from the to- 

 pographer, who maps the relief of land and adjacent sea 

 bottom; from the biologist and the paleontologist, who find in 

 the distribution of organic species, living and extinct, the evi- 

 dences for or against former land connections; from the 

 anthropologist, who may be able to show how much migratory 

 man may have changed the original conditions of organic dis- 

 tribution; and from the geophysicist, who tells whether each 

 island-studded region is in gravitative balance with the rest of 

 the earth. The naturalists of the Netherlands have had the wis- 

 dom to explore their East Indian domain along all of these 

 lines, and have remarkably illustrated the power of co-opera- 

 tion. Twenty-five years ago there was a possibility that, in the 



