JURA. GEOLOGY. 209 



toward Scarba. Various large sections of considerable 

 portions are to be seen in these different mountains ; 

 from the examination of which, evidence is afforded 

 of the stratified position of the quartz rock, even more 

 clear than that arising from the general view of the 

 island already given. The same places offer a ready 

 means of measuring the angles of elevation, by the 

 simple process of protracting the lines produced by the 

 section of the beds, on a graduated scale. As the 

 direction and position of the beds is best observed 

 on these mountains, the most certain estimate can 

 there also be made of their thickness. The quartz 

 rock being found at the foot of the whole, extending 

 to the shores of the sea on their western .boundary, and 

 maintaining its regular position to the summit, that 

 thickness may be discovered by a simple trigonome- 

 trical process, and is found to be about 2200 feet. It 

 is evident that the truth of this calculation rests upon 

 that of the assumed constancy of the angle 27, as well 

 as on that of the barometric measurement ; yet, under 

 any supposition, these beds will be found to form a 

 mass of considerable depth. They appear indeed rather 

 to affect a more horizontal position in the lower grounds, 

 and would therefore give dimensions greater than those 

 obtained from the foregoing computation. These dimen- 

 sions are countenanced by the corresponding scale of 

 those which form the mountains of Assynt; and when 

 compared with the extent of surface occupied by these, 

 points out quartz rock as a principal member among the 

 stratified substances of the primary class. 



The thickness here stated must not however be 

 supposed to extend through the whole range, which 

 reaches from the northern end of Scarba to the Mull 

 of Oe in Isla. On almost every occasion, stratified 

 rocks are found to vary in dimension as they proceed ; 

 gradually wearing out into a thin edge till they dis- 

 appear. Such deposits are, in fact, partial formations 5 



VOL. II. P 



