172 



Sketch of the Geology of the 



[Jan. 



recent formation, present the most simple illustration upon this point, 

 where we find the calcarious masses splitting under the action of the 

 sun's rays, crumbling and affording a scanty soil for foreign seeds float- 

 ed by currents to take root, and thus to extend the formation of the 

 soi], by the loosening power of their roots and the fall of their leaves.* 

 The heat of the sun alone may be considered, therefore, a powerful 

 auxiliary in the production of soils, and this influence is especially ap- 

 plicable to the Indian climate, where the dark hue of the rocks greatly 

 favours the imbibition of heat ; an observation similar to that which was 

 made by Link, with regard to the black slates of Pezo,t and subse- 

 quently by Humboldt, was demonstrated in reference to the black bare 

 rocks on the banks of the Oroonoko, whose temperatures were found to 

 be elevated during the day 341° F., and during the night 1 8° above 

 that of the atmospherej. In India, at the termination of the rains 

 in September, the porous stony masses must of course be saturated 

 with moisture, and when the rays of the sun excite their influence, and 

 are imbibed, the water must necessarily be vaporized, and disintegra- 

 tion of the rocky masses ensue. This effect of vapour must be consi- 

 dered as a powerful agent, although not so explosibly effective as the 

 freezing of water : 



" cum tristis hyems etiamnum frigore saxa 

 Bumperet."? 



Because, in one case the confined body has "some means of egress, 

 though far from free, while, in the other, the barriers m'ust yield on 

 every side to the overwhelming mass within. When the soil is carefully 

 examined, all the appearances confirm the idea of its derivation from 

 the rocky mass. It has a black colour, which it may in some measure 

 derive from particles of oxide of iron, which are occasionally observa- 

 ble in the porphyry where it has begun to decompose, and from the ve- 

 getable matter derived from the trees. No considerable portions of iron 

 have been detected in this neighbourhood, although the oxide of that 

 metal is found in considerable abundance among the ghauts, and is 

 smelted with some profit at the Mahabuleshwur hills||. In consequence 

 of the mixture of vegetable matter with this light rocky production, the 

 soil has been rendered fit for raising some scanty crops, which serve to 



* Dr. ft. Forster has well described the structure and mode of formation of coral islands, 

 in Cook's Voyage, of whose accuracy the present writer had ample proofs, while examin- 

 ing similar deposits on the coast of Sumatra. A fuller, but not more distinct, acoount of 

 coral islands is detailed in Beechy's Voyage. 



+ Link's Travels in Portugal, 8vo. 1801. p. 64. 



$ Humboldt's Pers. Nar. vol. v. pt. i: 26. 



F Virg. G. iv. 135. 



|! " Account of the convalescent station of Malcolm Pait on the Mahabuleshwur hills," 

 Bombay, 1830, pamphlet. The chemical nature of the ore is not stated in this publication, 

 but it is probably the magnetic iron ore, 



