﻿328 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JullC 25, 



of them ; their being commonly confined to the sides facing the ex- 

 terior of the island, although sometimes found on other and even on 

 all sides of a rock ; their great depth and regularity ; their general 

 coincidence with divisional lines ; their conformity to the course of 

 rain ; and their antiquity. It is this last circumstance which, pre- 

 senting at the outset a great difficulty, leads, on further considera- 

 tion, to what I consider the true explanation. That meteoric influ- 

 ences have been the great agents of erosion I have already suggested. 

 But the antique and permanent character which is impressed on the 

 great majority of the rocks, their vegetable coatings, the hardness 

 and sharpness of the external edges of the grooves, prove that the 

 rocks must have existed under very different conditions from the pre- 

 sent, to enable atmospheric forces to produce results of such magni- 

 tude. The considerations which have hitherto occupied us in the 

 concluding portion of this paper appear to me to indicate what these 

 conditions were. The composition and structure of the external rocks, 

 unveiled by the action of the sea on the beach, show zones of soft 

 rock*, rows of globular decomposing masses, and of harder ferrugi- 

 nous spheroids, &c, susceptible of being detached, and a general ten- 

 dency to perpendicular division. If, therefore, we conceive the ex- 

 ternal layer of the island, when it first became exposed to decomposi- 

 tion, to have resembled in character the zone that has been laid open 

 for our inspection along the beach, it is easy to comprehend how the 

 wasting away of the more decomposable parts might at last leave ex- 

 posed masses, including bands of the less stubborn material already 

 partially softened or disintegrated under ground, and that the action 

 of the atmosphere and rain-torrents would gradually excavate the 

 more yielding portions, until the solid remnants exhibited their pre- 

 sent shapes." 



II. Meteorological and Hydrological Influences. 



"We have necessarily anticipated much belonging to this head. 

 The mean annual temperature of the district is about 81° 25' (that of 

 the hottest and coldest months differing about 2° 76') ; the range 

 from 6° to 7° ; the fall of rain about 92 inches distributed over 

 every month in the year, with a considerable excess from October to 

 January. 



All the climatic forces which operate on the land are affected by 

 the two monsoons which prevail in the Indian Ocean and China Sea. 

 During the S.W. monsoon from April to October, the Straits of Sin- 

 gapore being to the leeward of the mountain-range of Sumatra, equable 

 weather is experienced in the district. Showers are frequent but 

 seldom long-continued, and the monthly mean of the thermometer 

 ranges from 81 '21 to 82*31 . 



During the N.E. monsoon from November to March, while the 



* Some rocks may be seen along the beach with chasms 2 or 3 feet wide, the 

 sides being quite hard and the bottom a soft decomposed substance. In such a 

 case a zone of rock differing in composition from that adjoining has evidently been 

 gradually decomposed and washed out. 



