SEIL. GEOLOGY. 129 



wacke schist, capable of being raised in coarse slates, and 

 scarcely distinguishable from that of Cumberland. It is 

 not however quarried, the manufactures of slates, in which 

 this island abounds, being solely occupied on the finer 

 schist. The quarries of Seil, like those of Eysdill, have 

 long been known, and their produce is the subject of a 

 very extended commerce. 



The lowermost beds of clay slate thus described, consti- 

 tute the middle ridge of the island. In arriving at the third 

 ridge, which lies to the eastward of this, a different set of 

 strata is seen, conformable to these in position and direction, 

 and, of course, immediately following them in the order of 

 superposition. These rocks are conspicuous at a distance, 

 not only from their grey colour, but from their unfriend- 

 liness to vegetation ; in consequence of which they are 

 seen protruding at the surface almost throughout their 

 whole extent. Besides their common division by parallel 

 seams, a portion of them is split into prismatic forms at 

 right angles to the beds, and thus attract notice at a con- 

 siderable distance among the darker rocks and more 

 verdant clothing of the clay slate. 



The most remarkable beds in this collection of strata, 

 consist of a mixture of pale grey compact felspar with a 

 greenish hornblende, extremely tough under the hammer. 

 This rock is of a light grey colour, and to its peculiar 

 fracture the prismatic aspect of the whole is owing. It 

 will be found to exist in many places hereafter to be 

 described, in a situation and with connexions similar to 

 those which it here possesses. That it may not be supposed 

 a member of the trap family, with which it is associated in 

 composition, it is necessary to say that the beds are regu- 

 larly interstratified with the surrounding rocks ; in which 

 respect it differs materially even from those masses of trap 

 that are occasionally found interposed among stratified 

 rocks so as often to put on the deceptive appearance of 

 true strata. It must be considered a member of a peculiar 

 series, in which chlorite and hornblende schists are the 



VOL. II. K 



