84 Dr. Davy on the Temperature of the Sea and the Air, 4' c - 



unfathomable ocean, and not surrounded by shoals, as islands 

 generally are. 



Very little is known respecting the climate of St. Helena. 

 More rain is said to fall at Plantation House, than in the 

 wettest part of Devonshire. The mean annual temperature 

 appears to be 64°, the thermometer rarely falling to 54°, and 

 seldom rising to 74°. For weeks together in the house, it has 

 been observed at 64°. The temperature of Longwood is con- 

 sidered a little lower than that of Plantation House, and that 

 of James's Town about 10° higher. 



This island is generally considered as of volcanic origin, and 

 all my observations confirm this opinion. The rock of which 

 the island consists exhibits great variety. In some places it 

 is very like basalt in texture, colour, and general character. 

 In other places it is extremely porous, vesicular and cellular, 

 indeed almost cavernous. Very often it has quite the appear-»- 

 ance of a slag. In a part of a rock remarkably cellular, sta- 

 lactites had formed exactly like some I had seen in the Mu- 

 seum of the Royal Society of Edinburgh, and which had been 

 brought by Sir George Mackenzie from Iceland, and were 

 decidedly of igneous origin. The substance of those I saw 

 at St. Helena was very like compact basalt. In some places 

 the rock showed a slaty structure, the imperfect strata ap- 

 pearing variously inclined. 



In point of disposition to decompose, the rock exhibits 

 much variety. In the same mass some part is entirely de- 

 composed and converted into clay, another part is undergoing 

 the change, and in different states of its progress, while ano- 

 ther part is not in the least altered. The decay of course is 

 greatest at the surface, where the rock is exposed to the at- 

 mosphere, but it is not confined to the exposed parts. The 

 clays which are formed from the decayed rock are of several 

 colours, of which brick red and pink red are the most com- 

 mon. The latter I suspect is produced by" manganese. I 

 did not see or hear of any beds of ashes or of pumice in the 

 island. 



Owing to the facility with which most of the rocks decom- 

 pose, the soil is in general deep. Even in the most barren 

 spots in the neighbourhood of James's town, there did not 

 appear to be any deficiency of soil, and I have no doubt that 



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