ISLA. GEOLOGY. 239 



present Isla, have been produced under circumstances far 

 different from any which could have occurred under its 

 existing form and connexions. 



IN order to render the geological description of Isla 

 more intelligible there is subjoined a general section at 

 rio-ht angles to the ridges.* Thus the inclinations of the 



o o o 



several rocks will be more visible, and the reader will at 

 the same time be able to trace those alternations which 

 form one of the most interesting circumstances in the 

 geology of the islands of this chain. As the quartz rock 

 constitutes the main body of the collective mass, I shall 

 commence with it, although not the lowermost rock ; 

 because the whole can thus be placed in a more luminous 

 and useful point of view. 



The general structure of the other islands of this chain 

 already detailed, will serve as a basis for the description 

 of Isla ; since the two principal ridges of hills, here called 

 the north-western, and the eastern ridge divisions, are simi- 

 lar to the main ridge of Jura. 



Wherever the cross section of these ridges is accessible, 

 the quartz rock is found occupying the same prolonged 

 direction as in Jura and Scarba, and generally dipping to 

 the eastward with corresponding inclinations. In Isla, as 

 in Jura, these vary in different places ; the variations 

 being equally remarkable in the former as in the latter 

 island. The variations in the direction appear to lie 

 between the north and the north-east, and the average 

 bearing may be taken as the mean between these, namely, 

 N. N. E. If it be taken still more to the eastward, it will 

 be found more conformable to that which has already been 

 stated as the probable bearing of the strata in the southern 

 parts of Jura. In the same way the horizontal incli- 

 nation changes from ten to fifty degrees, and in some rare 



* Plate XXII. fig. 4. 



