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 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. 1 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 crystalline 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 “ On the Geology of Sumatra,” by M. R. D. M. Verbeck. Geological 
Magazine , 1877. 
