NORTH UIST. GEOLOGY. 147 



or is mixed in the irregular manner already described, 

 both with the schist and with the gneiss by which it 

 is accompanied. It is to be found in some of the 

 low islands at the mouth of Kyleswiaveg, but is most 

 conspicuous and most easily examined in the hills them- 

 selves, appearing to terminate in gneiss on the shores 

 of Loch Maddy, as it ends in mere schist on the sea 

 shore. It is difficult to render intelligible by description, 

 the aspect of a rock or mixed mineral, but this will 

 be most easily understood by referring the several va- 

 rieties to those better known rocks which, in its simplest 

 state, it most resembles. At one extreme it appears to 

 be a lead coloured compact felspar, characterized by 

 extreme toughness, and often by a twisted texture ; 

 displaying at the same time a mixture of lighter and 

 darker tints. When straight and even in the fracture, 

 it resembles on a superficial view a siliceous schist. 

 Occasionally it is interspersed with grains of transparent 

 quartz, in which case small fragments would readily 

 pass for specimens of hard felspar porphyry. At times 

 it is so penetrated by quartz diffused throughout in a 

 sort of regular gradation, that the boundaries of the 

 two are not visible. A new set of varieties is formed 

 by similar admixtures of common felspar ; while other spe- 

 cimens present mixtures of felspar and quartz, or felspar 

 and hornblende,' or hornblende and quartz, or of all the 

 three substances united with the base. It passes in these 

 cases into granite, or into hornblende schist ; or, by 

 the ultimate exclusion of argillaceous schist, into gneiss. 

 Portions also of common compact pale felspar are 

 found in it, or beds in which this rock forms a basis 

 slightly modified by the occasional mixture of other 

 ingredients. Such are the characters of the different 

 portions of this rock, for which no name has yet been 

 assigned, and of which I have no where else met with 

 any resemblance. It may perhaps be considered as 

 intermediate between gneiss and compact felspar; but 



