150 TORSA. GEOLOGY. 



unworthy of further investigation ; since, if established, 

 it may tend to elucidate many circumstances in the alter- 

 nations of softer strata with trap rocks of a hard structure, 

 which have been considered at variance with the sup- 

 posed igneous origin of these substances. 



The last circumstance worth noticing in Torsa, is another 

 peculiarity in the weathering of some of the trap veins, 

 which, although it occurs also in Seil, was deferred, as 

 the instances are here much more remarkable. Though 

 here called trap veins, for want of a more definite term, 

 and though generally known to mineralogists by the 

 name of greenstone, they consist principally, often 

 entirely, of compact felspar or of clinkstone ; their colour, 

 when there is any, being derived from iron, not from an 

 admixture of hornblende. 



In one of the instances selected for description, the 

 rock, in its original and unaltered state, is white and 

 compact. By degrees, as it approaches the surface, it 

 becomes of a rusty brown, and at length that colour 

 as gradually disappears, the external crust becoming 

 white and pulverulent. In a second, the original rock 

 is of a lead blue, and undergoes the same alterations. 

 The changes which take place in the third and last 

 instance that I shall here notice, are much more intri- 

 cate and less easily explained. The vein in this case 

 exhibits the following succession of colours ; brown, 

 buff, white, lead blue, brown; the first and last named, 

 occupying the two external sides where it has been 

 exposed to the weather. 



The change from blue to brown which occurs in the 

 second case is easily understood, since it results from 

 the conversion of the protoxide of iron into rust. As 

 the same rust is formed on the weathered part of the 

 white rock described in the first example, it is evident 

 that this rock also must contain iron, which is probably 

 in the state of carbonate, and is afterwards converted into 

 the brown substance by the absorption and combination 



