1839] 



some other portions of Mysore. 



113 



supposes them to be produced by the passage of rarified air through the 

 fissures ; and he seems to think that the impulse against the elastic 

 scales of mica which intercept the crevices may contribute to modify 

 their expression. The epidemic fevers which prevail on the Orinoco, 

 the natives refer to the noxious exhalations that arise from the bare rocks 

 of the rapids. This Humboldt remarks is worthy of attention, on ac- 

 count of its being connected with a fact that has been observed in seve- 

 ral parts of the world, although not sufficiently explained. Among the 

 cataracts and falls of the Orinoco, the granite rocks, whenever they are 

 periodically submerged, become smooth and seem as if covered with 

 black-lead. The same appearance is presented at the cataracts of Syene, 

 as well as those of the Congo. This black deposit, according to Mr. 

 Childi-en's analysis, consists of oxide of iron and manganese (so says 

 Mr. Macgillivray, the translator of Humboldt), to which some experiments 

 of Hum.boldt induced him to add carbon and super-carburetted iron. Mr. 

 Macgillivray goes on to say that this phenomenon has only been ob- 

 served in the torrid zone in rivers that overflow periodically, and are 

 bounded by primitive rocks. M. Humboldt supposes it to arise from the 

 precipitation of substances chemically dissolved in the water, and not 

 from an efflorescence of matters contained in the rocks themselves. 

 The Cauveri overflows periodically, and its waters pass through a most 

 jungly tract of comitry. 



There is another circumstance connected with the river, which is 

 this — that although there is often but little water, thus leaving bare 

 the rocky masses, water will most likely be near the surface of the 

 ground all over the island, and lead to dampness and exhalation. In 

 the mornings and early part of the forenoon, a dense vapour covers the 

 fort and island, when the neighbouring country is perfectly clear. All 

 valleys and low positions in Mysore, are more or less unhealthy, and are 

 much hotter in the day and colder at nights. The mean annual tempe- 

 rature of Seringapatam is two degrees higher than Bangalore — the ave- 

 rage mean lowest of Seringapatam about 63— that of Bangalore 69— the 

 average highest of Seringapatam 90— that of Bangalore 81 — a difference 

 of 27 degrees in the one, and only 12 in the other. This alone might 

 account for the prevalence of the fever, and the other circumstances, 

 previously mentioned, explain its malignity. 



The rocks between Seringapatam and Mysore are chiefly igneous, trap 

 and porphyry— hornblende seems the most common, and has often on 

 the surface a most variolated aspect. There are few rocks besides the 

 hornblende, not including the usual granite and gneiss, to be met with, 

 but many of the buildings on the road side consist of a rock of a por- 

 phyritic character, purple brown compact felspar, with white or reddish 



