496 RUM. GEOLOGY. 



of Loch Scresort. It is, I believe, universally supposed, 

 that in all the columnar veins the direction of the columns 

 is at right angles to that of the vein. In this, their direc- 

 tion is parallel to it, and their position horizontal. The 

 vein itself is of considerable size, and is divided into 

 columnar masses of an irregular shape, of which four, 

 five, or more lie laterally in its thickness ; while, in its 

 direction, they are separated into rude joints of irregular 

 lengths.* 



Having described these most prevalent of the trap rocks 

 found in Rum, it is still necessary to enumerate one or 

 two which cannot be referred to either, and which bear a 

 greater resemblance to the stratified traps of Sky and 

 Canna, than to those already examined. They are found 

 on the north-western side of the island. One of these is 

 seen in detached masses on the north-western summit of 

 Oreval, from whence it extends towards Kilmory ; being 

 clearly incumbent both on the syenite and on the sand- 

 stone, according to the different places which it occupies 

 among the hills. It has a flat surface and a prismatic 

 fracture ; bearing, in these respects, no resemblance to 

 those at the eastern side of the island, and having no 

 other common feature with them but that of containing 

 augit as a part of its composition. The other is incumbent 

 on the summit of Greg na Stiarnin, reposing on the sand- 

 stone ; and its abrupt face, distinguished by the name of 

 Scuir more, is well known as the repository of the helio- 



* The rarity of this occurrence induces me to mention a similar 

 instance which is found on the west side of the Mull of Cantyre. In 

 this case the columns are also parallel to the vein but in a vertical 

 position. 



It has been said that the transverse disposition of the columns in trap 

 veins originated from the same cause as their vertical one in ordinary 

 cases ; namely, from the cooling having commenced at that part which 

 in both cases must be considered as the surface. The present instance 

 will not admit of this explanation, nor is it more applicable to the masses 

 of columnar rock on the eastern side of Stafia. 



