TORSA. GEOLOGY. 151 



of water. In the third case, the interior of the vein 

 presenting both blue and white tints, it follows that 

 both the carbonate and protoxide are combined with the 

 earthy basis ; a circumstance which is proved by the 

 change to brown taking place gradually at the outer 

 side of each. If however we may trust to this natural 

 analysis, the proportion of iron in the blue part of the 

 rock exceeds that in the white ; as the brown colour 

 is far more intense on this side of the vein ; a consequence 

 that might easily have been foreseen from the appear- 

 ances and nature of these different parts. The gradation 

 occurring between the blue and white, would seem to 

 indicate that the protoxide had in some way been con- 

 verted into the carbonate ; although it is possible that 

 both these combinations of iron may exist in the rock 

 independent of each other. 



The last change which occurs in all these cases, is 

 the disappearance of the brown colour, a pure white 

 and powdery felspar forming the outermost superficies. 

 The iron has here disappeared, and it is probable that 

 this ultimate effect results from its supra-carbonation and 

 consequent solution in the atmospheric water. This 

 change is not uncommon in other rocks, and I have on 

 different occasions remarked that it takes place in quartz 

 rock, as well as in those combinations of quartz and 

 felspar that occur in the sandstone series of Sky ; which, 

 although of a very dark colour, become bleached to 

 absolute whiteness by exposure. As the change from 

 white to brown is also not unusual, even in the most 

 compact and whitest varieties of quartz rock, it is also 

 probable that even these contain iron in the state above 

 described ; little as it would be suspected by merely exa- 

 mining their external characters. 



