260 FLODDA. GEOLOGY. 



rocky islets, all resembling it in composition. There 

 is a perfect natural section on the side next to Rasay by 

 which its structure is completely displayed. 



The whole island is formed of the graywacke schist 

 and red sandstone already described as found near 

 Brochel Castle, the former however bearing a very con- 

 spicuous proportion to the latter. As it is similar to this 

 part of Rasay in composition, so it corresponds in the 

 inclination and direction of the strata; but the angle 

 of elevation is lower and does not seem to exceed ten 

 degrees, while in some places it is as low as five. As the 

 direction of the strata is similar in both the islands, 

 and the position of the sandstone in Rasay is regular, 

 it might have been expected that by prolonging the 

 outer line of the latter, the whole mass of sandstone 

 in both would have been determined by a rectilinear 

 boundary. This however is not the case, since the point 

 of gneiss already mentioned protrudes across the line which 

 should have been here uninterrupted. It is not difficult to 

 comprehend the cause of this, if we advert to the proba- 

 bility that the sandstone was deposited on an uneven sur- 

 face of gneiss, and that, the section parallel to the horizon, 

 produced by the ordinary wearing of the surface, would 

 consequently leave a protuberant mass of this rock 

 trespassing beyond its linear boundary. The same effect 

 would follow if we conceive the gneiss to have been 

 elevated after the deposition of the sandstone. This 

 circumstance will explain the irregularities that take 

 place in Rasay at the common boundary of these rocks, 

 which I forbore to notice in describing that island, because 

 the appearance was neither explicit nor the cause obvious. 

 The graywacke that lies under the sandstone, forms 

 a series of beds with a schistose fracture parallel to 

 their planes of stratification, being immediately followed 

 by that rock, without any intermediate conglomerate 

 as in Rasay. I did not here observe it alternating with 

 the sandstone ; which is nevertheless to be expected, 



