Dr. Mac Culloch on Staffa. 507 



rocks. It is most easily seen at that side of the island which 

 faces lona, and on the summit of the cliffs of a semicircular bay 

 opening in that direction. The bed is here broken at the edge of 

 the cliff, so as to expose its whole thickness for a considerable 

 extent. But the same appearance may also be observed immediately 

 above the ordinary landing place, where the bed has also been 

 broken. The stones which it contains are all rounded, and of va- 

 rious, often considerable dimensions, and they exhibit specimens 

 of granite, gneiss, micaceous schistus, quartz, and red sandstone. 

 Together with these, are some rolled pieces of basalt. 



Here then is a circumstance in the mineral history of Staffa, 

 adventitious it is true, but involving difficulties of no small impor- 

 tance. If w^e cast our eyes on the map, (PI. 35.) we shall perceive 

 that it is embayed in a large sinuosity, formed in the island of 

 Mull, and nearly enclosed on the opposite side by lona, and the 

 Treshanish islands. Beyond the latter, a second line is drawn by 

 Tirey, and Coll ; while to the north, but at a greater distance, are 

 placed the islands of Muck, Rum, Egg, Canna, and Sky. The 

 wliole island of Mull, with the exception of the Ross, is of a trap 

 formation, containing however some partial tracts of sandstone 

 and other rocks which I need not notice. The islands of Ulva 

 and the Treshanish, with their dependent rocks, are also of trap 

 formation. So are the islands which lie to the north, and which 

 I have enumerated above. lona however, together v/ith Coll and 

 Tirey, consists principally of gneiss and mica slate traversed by 

 granite veins, rocks which also form the chief parts of the coasts of 

 Lorn, Appin, Morven, and Ardnamurchan. 



It is to the former then, that we must look for the origin of the 

 rolled stones which cover Staffa, if, limiting the great operations of 

 nature by our own narrow views, and the ages which have con- 



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