1SLA. GEOLOGY. 243 



which this rock affords of the repeated alternation of 

 graywacke and clay slate within a very limited space, 

 it is interesting in another way, as presenting a criterion 

 by which in many cases, where no other is attainable, 

 we are enabled to ascertain the position of beds of clay 

 slate, often rendered uncertain by the facility with which 

 the plane of the schistose division may be mistaken for 

 that of the bed. 



The second of these circumstances attending the clay 

 slate, is its alternation with gneiss. This was already 

 pointed out in North Uist, but the appearances there 

 are by no means so obvious as in Isla. The gneiss is 

 to be seen skirting the shore from Octofad to the Rinns, 

 and beyond that point. In many places the position 

 of both the rocks is regular, and conformable to the 

 general position of the other strata, while the gneiss is found 

 in large masses, lying either above, or below the schist, or 

 in alternate order. In other cases the clay slate is con- 

 fused and irregular, and the gneiss also is found irregularly 

 interchanged with it. In no instance was any transition 

 visible between the two, the boundary being strongly 

 marked, and the schist as well characterized as in any 

 other place. As this rock occupies a space of some miles, 

 it cannot be the subject of mistake, either as to its quality 

 or position ; nor can it be considered as an anomalous and 

 accidental body. Its composition is peculiar, but has 

 already been noticed in a cursory manner on former occa- 

 sions, particularly in lona and in the Long isle. It con^ 

 sists chiefly of felspar, of a red colour, containing distinct 

 aggregations of fine granular hornblende disposed in a 

 scaly manner, or else similar scales of argillaceous schist ; 



lamina in a different order from the rest; but when the whole bed is 

 forcibly divided, both schists yield together. A similar appearance was 

 pointed out in the sandstone of Strathaird in Sky, and an attempt was 

 made to account for it by the same supposition which is applicable to 

 this, namely, by considering it a modification of the concretionary struc* 

 ture. 



