ISLA. GEOLOGY. 241 



quently the uppermost in position. Hence it might have 

 been concluded that the quartz rock was the lowest sub- 

 stance, and that the great mass of Jura was formed of it, 

 while the schists succeeded ; an error easily committed by 

 limiting the investigation to that island. The situation of 

 the schists in Isla, shows the fallacy of this supposition ; 

 while it presents in an infinitely stronger point of view 

 than the remainder of the chain, the alternations of the 

 quartz rock with these. 



The south-western district consists principally of clay 

 slate ; but as it is in different places found alternating 

 with thin beds of quartz rock, it is not possible to define 

 its geographical boundary. Different observers would 

 perhaps assign a different one, and perhaps all would be 

 equally wide of the exact truth. Fortunately this uncer- 

 tainty does not affect the geological inferences to be 

 drawn from the facts ; while, in a topographic view, it is 

 of no importance. If an irregular line be drawn from 

 Ardneave to Octofad, it will leave but little quartz rock on 

 the western side ; and at some distance from this line, the 

 whole will be found to consist of clay slate ; with an 

 important exception, that will be noticed in its proper 

 place. The bearing of this schist is in general con- 

 formable to that of the former rock ; the irregularities 

 which occur being unimportant, and depending on those 

 partial causes which it is now unnecessary to repeat. Of 

 many places, where both the northern direction and the 

 eastern inclination of the clay slate can be distinctly seen, 

 I shall only point out Sanig, where the appearances are 

 quite satisfactory, and where the position is strictly con- 

 sonant to that of the leading ridge of Jura and to its 

 prolongations both on the western and eastern sides 

 of Isla. It is difficult even to conjecture the thickness 

 of the collective mass on this side of the island ; since 

 the angle of inclination is too irregular to admit of any 

 computation from a comparison of the extent with the 

 visible' portions ; but assuming that thickness only which 

 VOL. n. R 



