182 Dr. Mac Cul loch's Supplement to the 



prehended as a whole, is not less symmetrical than that of the faces 

 of StafFa, while at the same time they far exceed it in grandeur as 

 well as in absolute magnitude. Their height reaches from 200 to 

 300 feet and upwards, a dimension, however large, not sufficient to 

 overpower the due proportion which should exist between the 

 aggregate structure and the parts of which it is composed, since the 

 magnitude of the columns is proportioned to their height, and the 

 total effect therefore similar to that of Staffa, where the proportions 

 are so nicely adapted for beauty. 



With respect to the composition of this variety of trap, there is 

 necessarily some uncertainty, since the great extent of it, as well as. 

 the inaccessible nature of most parts, renders it utterly impossible 

 to examine it throughout. We also know that the various mem- 

 bers of this family are often found irregularly intermixed, so that 

 to have ascertained the composition of one portion of a mass, gives 

 us no assurance that we have made ourselves acquainted with that 

 of the whole. Yet I am inclined to think that the greater part 

 will be found to consist of a substance analogous to greenstone, 

 in which augit occupies the place of hornblende, a rock of great 

 frequency in Scotland, and often, perhaps generally hitherto con- 

 founded with common greenstone, unless in a few such remark- 

 able cases as that of Rum, where the substances are too distinct to 

 admit of mistake. It may be called augit rock, without introducing 

 any confusion into mineralogical nomenclature. 



For the sake of topography I must here mention a small mass of 

 trap, lying on a part of the coast of Sleat not easily visited, and 

 omitted in the original paper. It occupies a projecting point south 

 of Talivil, where its place has been marked in the amended map. 

 It covers a space of about a mile in extent, lying over the red 

 sandstone. It is rudely columnar and slightly porphyritic, and 



