RASAY. GEOLOGY. 49 



that occupies the northern part of the island already 

 described. With respect to its structure, it will be suffi- 

 cient to remark in general, that it consists in most cases 

 of quartz sand cemented by red clay, with a small propor- 

 tion of mica ; being obviously formed from disintegrated 

 granite or gneiss. In a few instances the beds appear 

 highly condensed and crystalline, being composed of quartz 

 and red felspar intimately united; thus presenting a fur- 

 ther illustration of their origin, the latter having in the 

 progress of change become the red clay which forms 

 the cementing medium in the predominant specimens. 



In terminating the history of this sandstone it may be 

 remarked, that I have thought it unnecessary to specify 

 the situations where a coarser gravel stone occurs, as it 

 seems in all cases an accidental feature no way affecting 

 the geological or mineral history of the rock. It happens 

 occasionally in the progress of a single bed that the tex- 

 ture will thus vary, while in other cases the change from 

 fine to coarse takes place in irregular alternation. Varia- 

 tions of colour are also here to be observed, and, like the 

 changes of texture, they are sometimes lateral, sometimes 

 alternate. They are not very uncommon in this rock, 

 and will be shown hereafter to exist in Sky in a very con- 

 spicuous manner. 



If there are any other portions of red sandstone to be 

 found in Rasay, a circumstance not improbable, as many 

 scattered pieces of the uppermost strata are to be seen 

 in different places, there will be no difficulty in referring 

 them to the fundamental mass ; while the disturbance 

 visible in some parts of the island will, together with the 

 prevalence of overlying porphyry, be sufficient to account 

 for their existence. 



Peculiar circumstances in the disposition and structure 

 of this island render it difficult to determine precisely 

 the beds which lie next in order to the red sandstone, 

 but by combining analogy with observation they may 

 perhaps be discovered. 



