CROULIN ISLES. GEOLOGY. 



81 



THE CROULIN ISLES.* 



WITH the view of extending the comparison between 

 Sky and the mainland, these have already been cursorily 

 mentioned in the general remarks on the Trap islands; 

 their vicinity to the shore affording the means of establish- 

 ing the geological connexion of that island and the con- 

 tinent. 



The southernmost and largest of the group appears to 

 be about a mile in length, and is known by the name 

 of Croulin more ; the next is Croulin beg, and the north- 

 ernmost is called St. Rufus's island. The structure of 

 the whole three is so similar, that it is unnecessary to dis- 

 tinguish them in description, or to give a detailed account 

 of the rocks of which they are composed; since the mine- 

 ral characters of these have been amply described already. 

 They are the same substances which form the upper line of 

 the sandstone in Sky, namely, hard red sandstone, schistose 

 sandstone, and graywacke schist. The bearing of the 

 strata is, like those of Sky, north-easterly, the dip being 

 also to the north-west as in the uppermost beds of that 

 island, and the angle of inclination lying between twenty 

 and thirty degrees. They are traversed by veins of trap. 



If the position of the strata in these islands be com- 

 pared with those of Longa and Scalpa, it will be seen 

 that they also coincide in bearing with the two latter, 

 at a distance of six or eight miles, a deep sea intervening ; 

 from which it may be concluded that these are all portions 

 of the same line of strata. This was already noticed in a 

 cursory manner in treating of Scalpa; and since every 

 thing which appeared necessary respecting those general 

 connexions of the sandstone has already been said, it is 

 here unnecessary to enter into any further discussions. 



VOL. n. 



* The Red isles. See the Map of Sky. 



