TRAVELS IN BRAZIL. 93 



The latter are irregularly scattered, some of them 

 very small, others several lines long and broad, or 

 sometimes run together into irregular hollows. 

 Near them the colour of the basalt is either quite 

 the same, or declines into a yellowish brown, which 

 is probably caused by a decomposition, partly of 

 the iron, and partly of the olivin ; the latter is 

 in great quantities, and of different dimensions, im- 

 bedded in the mass of the basalt ; when fresh 

 broken it is shining, and of a light 'olive green. 

 In a weak state of oxydation its imperfectly 

 foliated fracture separates, and such pieces are 

 iridescent ; it shows then principally a dark yel- 

 low or brown colour, in which the lustre and 

 transparency are lost. The phenomenon of the 

 attraction and repulsion of the magnetic needle 

 is very evident in the basalt of Madeira ; it 

 often approaches the wacke ; its vesicular ca- 

 vities are then larger, often above an inch in 

 length, and sometimes filled with a bluish earth, 

 but generally lined with a pulverulent coating. In 

 this softer stone are imbedded grains of olivin, 

 often of a yellowish brown colour. At a con- 

 siderable elevation, particularly on the surface of 

 the ground, the rock consists entirely of wacke. 

 It is of an ash and bluish grey colour, mingled 

 with small, scaly, black points. The stratification 

 is very apparent in it ; the layers are generally 

 horizontal, and their hardness and weight is less. 

 The phenomenon of polarity was more obvious in 



