CHAP. XVII BORNEO, JAVA, AND THE PHILIPPINES 



384 



and modification of species must have been the repeated 

 changes from cold to warm, and from warm to cold con- 

 ditions, with the migrations and crowding together that 

 must have been their necessary consequence. But in the 

 lowlands, near the equator, these changes would be very 

 little, if at all, felt, and thus one great cause of specific 

 modification would be wanting. Let us now see whether we 

 can sketch out a series of not improbable changes which 

 may have brought about the existing relations of Java and 

 Borneo to the continent. 



Past Geographical Changes of Java and Borneo. — 

 Although Java and Sumatra are mainly volcanic, they are 

 by no means wholly so. Sumatra possesses in its great 

 mountain masses ancient crystalline rocks with much 

 granite, while there are extensive Tertiary deposits of 

 Eocene age, overlying which are numerous beds of coal 

 now raised up many thousand feet above the sea.^ The 

 volcanoes appear to have burst through these older 

 mountains, and to have partly covered them as well as 

 great areas of the lowlands with the products of their 

 eruptions. In Java either the fundamental strata were 

 less extensive and less raised above the sea, or the period 

 of volcanic action has been of longer duration ; for here no 

 crystalline rocks have been found except a few boulders of 

 granite in the western part of the island, perhaps the relics 

 of a formation destroyed by denudation or covered up by 

 volcanic deposits. In the southern part of Java, however, 

 there is an extensive range of low mountains, about 3,000 

 feet high, consisting of basalt with limestone, apparently 

 of Miocene age. 



During this last named period, then, Java would have 

 been at least 3,000 feet lower than it is now, and such a 

 depression would probably extend to considerable parts of 

 Sumatra and Borneo, so as to reduce them all to a few 

 small islands. At some later period a gradual elevation 

 occurred, which ultimately united the whole of the islands 

 with the continent. This may have continued till the 

 glacial period of the northern hemisphere, during the 



1 "On the Geology of Sumatra," by M. R. P. M. Verbeck. Geological 

 Magazine, 1877. 



