TRAVELS IN BRAZIL. 95 



diction with the hardness of the stone. The rela- 

 lation of its density, together with the natron it 

 contains, is the most important cause of the great 

 affinity with the water of the atmosphere. It is 

 known that no kind of rock attracts the latter so 

 strongly and so continually as the basalt, which is 

 so remarkably compact ; for this reason we so 

 often see its summits veiled in thick clouds, and 

 marshes in its vicinity. The basalt, too, in conse- 

 quence of its disposition to assume columnar flat, 

 and spherical forms, is more exposed than any other 

 rock, on a thousand points, to the influence of the 

 atmosphere. Hence, and still more by its remark- 

 able composition of silex, clay, lime, talc, natron, 

 oxyd of iron, nay, even muriatic acid, the basalt, 

 more than any other kind of rock, appears like a 

 great voltaic column. This comparison seems 

 more just, if we consider the composition of the 

 single strata of the flotztrap mountain ; yet it is 

 still worthy of remark, that the massy undetached 

 basalt related to the amygdaloid, or the wacke, 

 decomposes more readily than that which is se- 

 parated into pillars, and which is more crystalline. 

 From one of the highest points of the island, 

 which is covered with the Pinus canariensis of 

 Smith, and with ferns, we descended in the evening 

 through several deep ravines, and a thick grove of 

 beautiful laurels and chesnuts, to the solitary 



* Kennedy in Gilbert's Annals, vii. p. 426, 



