94< 



TRAVELS IN BRAZIL. 



this wacke than in the basalt which lay lower, 

 which coincides with the observation made by 

 Giesecke, according to which the basalt at ele- 

 vated points is more magnetic than that in lower si- . 

 t nations.* That the basalt sooner acquires polarity 

 in elevated places, that is, where it is more insu^ 

 lated from the soil, is to be ascribed to the same 

 cause ; in consequence of which every stone sus- 

 ceptible of magnetism, even the magnetic iron- 

 stone itself, does not become magnetic till it is 

 brought up into the air and light ; the iron 

 weathercock till it is placed on the steeple, and 

 every rod in general, till it is set upright. In 

 elevated places exposed to the sun, and where the 

 basalt is covered with the mould, ferruginous clay 

 is found in brownish red masses, with granular 

 fracture, sometimes hard, sometimes half hard. 

 Friable brown points, probably of clay iron-stone, 

 and delicate sparkles of pinchbeck brown mica, 

 are scattered in it. The olivin is decomposed 

 into a yellowish brown, mass, in which, however, 

 the cleavage is still to be recognised. These 

 red spots of ferruginous clay are distinguish- 

 able from the sea, and heighten the variety 

 and vivacity of the picture presented by the 

 lovely mountain island. For the rest, the consi- 

 derable decomposition of the basalt, which is ob- 

 servable here also, is only apparently in contra- 



Edinburgh Philosophical Journal, 1821, p. 221. 



