240 ISLA. GEOLOGY. 



instances, reaches to seventy ; while in one or two, the 

 strata even become horizontal, or incline at length 

 to the westward. But these are anomalies of which a 

 very satisfactory explanation may be seen in this island, 

 as well as in Scarba ; the undulations of the strata which 

 cause them, being very remarkable on its eastern side. 

 Among other places, they are very visible at Sanig. But 

 these undulations, and consequent anomalies of inclination, 

 are generally found on the edges of the main ridge, the 

 dip of which is consistently to the eastward. The true 

 average of that dip is not to be assigned, nor is it essential. 

 From this it is apparent, that if the beds of quartz rock 

 which form the leading ridge of the three islands, be con- 

 sidered according to their length, they will be found to 

 undergo a lateral undulation ; while the variation in the 

 quantity of inclination at different points, is the result of a 

 second curvature of the plane at right angles to the 

 former. It is only by observations so extended and com- 

 pared, that the true bearings and connexions of the ancient 

 stratified rocks can be perceived. Limited observations 

 on those subjects are apt to lead to erroneous conclusions. 

 The general appearance, alternations, and mineral struc- 

 ture of the beds of this rock have been already described 

 so fully, in treating of Jura,, that it would be mere repe- 

 tition to enter into it again, as there is a perfect identity 

 between them in both islands. I may only remark, that 

 at Macarthur's head it contains felspar in considerable 

 proportion, and that it there also contains pyrites. Hence 

 it acquires on weathering a reddish or brown colour ; an 

 appearance also very remarkable in Loch Eribol, and at a 

 distance very deceptive, as it is easily mistaken for granite 

 on a superficial view. In this headland, some beds of 

 micaceous schist are also found interposed among the 

 general mass of quartz rock. It w r as also shown, that on 

 the eastern side of Jura, Lunga, and Scarba, the quartz 

 rock alternates with clay slate and micaceous schist ; the 

 main bodies of which lie on that face, and are conse- 



