CHAP. XVII.] 



BORNEO AND JAVA. 



359 



important an agent in producing extinction and modification of 

 species must have been the repeated changes from cold to warm, 

 and from warm to cold conditions, with the inevitable migrations 

 and crowding together that must have been their necessary con- 

 sequence. But in the lowlands, near the equator, these changes 

 would be very little 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 Geogra;pMcal 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.i 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 product of their erup- 

 tions. 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 cr}/stalline rocks have 

 been found except a few boulders of granite in the western 

 part of the island, perhaps a relic 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 hemi- 

 sphere, during the severest part of which a few Himalayan 



1 **0n the Geology of Sumatra," by M. R, D. M. Verbeck. Geological 

 Magazine, 1877. 



