40 Dr. Mac COlloch's Sketch of the 



described. The absolute identity of its aspect, composition, and mode 

 of weathering, might perhaps be sufficient to prove the identity of 

 its nature, but this is put out of all doubt by finding that it is here 

 followed by the same set of beds which succeed to it in that place 

 where the sandstone precedes it, and of which the continuity can 

 be traced between the two points. The drawings (PI. 2. fig. 1.) 

 which accompany this paper, will explain better than words this 

 very essential circumstance in the geological history of Sky. 



There is here an opportunity of tracing by a very perfect natural 

 section the change which it undergoes between the very regular 

 beds which lie near it on the one hand and the irregular surface of 

 syenite with which it is in contact on the other. At this surface it 

 bears no marks of stratification, but is an irregular and almost shape- 

 less mass, while near the former beds, which also consist of limestone, 

 it becomes first vertical and gradually more regular, till at the end its 

 general bearing, although much deformed by counter fissures, partakes 

 most decidedly of the general inclination of the stratified rock, which 

 is here about 25°. In geographical distribution it may be here traced 

 in two divisions, separated from each other by syenite and inter- 

 sected by trap veins, both of these divisions extending towards 

 Broadford and uniting into one scattered and irregular mass about 

 three or four miles short of that place. The uniformity of its 

 character, as well as its continuity throughout this tract, is such as to 

 leave no doubt that the marble of Broadford already mentioned, 

 and which I shall hereafter more fully describe when I speak of 

 individual minerals, is a continuation of this portion of the limestone 

 beds. When it has once quitted the sandstone that rock is no longer 

 seen, but the whole remaining portions of the limestone, occupy- 

 ing a great space from the hills which skirt the eastern side of 

 Strath to the foot of the syenite mountains on the opposite side, is 



