269 



GEOLOGY. 



By C. "W. Andrews, B.Sc, F.G.S. 



Prom the description of the physical features of Christmas Island 

 given at the beginning of this Yolnme, it will be gathered that the 

 island may be considered as probably an ancient atoll which has 

 been raised to a considerable height above the level of the sea. 

 The chief reasons for so regarding it are, firstly, the general form 

 of the plateau with its outer border of slightly higher land, and 

 secondly, the nature of some of the rocks occurring on the more 

 elevated points. In the present section a brief sketch of the 

 general geological structure is given, a fuller account being deferred 

 till the nature of the rocks, especially of the later limestones, has 

 been more completely investigated, both with the microscope and 

 by chemical analysis. 



The difficulties in the way of a detailed examination of the 

 geology of the island were considerable. The density of the 

 vegetation both rendered locomotion slow and difficult, and, at 

 the same time, concealed much that it was desirable to see (Fig. 1). 

 Moreover, the want of sufficient men for carrying water made it 

 impossible to stay more than a day or two in places remote from the 

 settlement or from the streams on the east coast. In consequence 

 of this the south of the island was not fully examined ; but since 

 in those parts which were traversed it was like the northern and 

 eastern regions, and since the appearance of its seaward slopes was 

 the same as that of the other coasts, it seems improbable that the 

 structure of this region diJSers in any important respect from that 

 of the rest of the island. Much, however, remains to be done, and 

 now that several roads have been cut and a number of men could 

 be employed, further exploration would be comparatively easy. 



One of the chief objects of the expedition to Christmas Island 

 was to find out whether its structure would throw any light upon 

 the vexed question of the nature of the foundations of atolls. The 

 various views that have been put forward in this controversy are 

 so well known that it is unnecessary to give any account of them 

 here. From the description which follows, it will be seen that at 

 Christmas Island at least we do not find the great thickness of 

 reef -limestone required by the Darwinian theory of atoll-formation, 

 and although there may be some evidence that subsidence did occur 

 in the earlier history of the island it is clear that it was neither 

 continuous for any long period nor of any great extent. It may, 

 of course, be objected that Christmas Island was never a typical 



