486 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
vitrifaction. The conchoidal fractures exhibit a perfect bottle-glass appearance, 
very similar to that of augite melted in a crucible. Close inspection with a 
lens, however, shows on the crystalline surfaces a cupped appearance, quite 
similar, though not so minute, as that which in decomposed glass was shown 
by Sir Davin Brewster to be the cause of the iridescent colouring. Closer 
inspection still, demonstrates that these cup-shaped hollows had been the recep- 
tacles of little spheroids of felspar, which may be seen arranged in layers in 
portions of the mass of the stone. I call the spheroids /e/spar merely from the 
lustre of their cleavages, and from the presence of alkalies,—as shown by 
analysis. 
Two questions at once connect themselves with these “rough, rounded, and 
broken lumps,” which helped to choke up a former volcanic orifice. 
The first— Whence came they, or what were they broken from ? 
The second—Do they, in themselves, give us any information as to the 
amount of heat to which they had been subjected in the process of their vitrifi- 
cation ? 
These two questions may resolve themselves into the more general one—Js 
there any liklihood that we may be able—by their recognition as components of 
a well-known stratum, or by the amount of alteration which has been effected 
upon them by heat—in any measure to arrive at an estimate of the depth at 
which the volcanic turmoil originated ? 
Such a question is too wide a one to be entered upon in its entirety here; 
but although a consideration of it, in the two directions indicated above, may 
lead us but a small extent to any definite answer, yet to that small extent it 
unquestionably must lead us. 
All fragments must have been brought by an outflow from beneath the 
parent rock from which the fragments were torn: the amount of change, if 
incomplete, is the register of the highest degree oe the heat, which was the 
active agent of the change. 
Asking ourselves in this instance: Whence came these lumps of augite ? 
we are able to reply unhesitatingly,—from no part of the Old Red;—no formation, 
not even the chalk, could have less claim in ‘ztse/f to an augitic mineral. 
Inferior to the Old Red, we have in this part of Scotland gneisses and flaggy 
schists, thrown into wavy folds, where they cross the breadth of Sutherland. 
These are probably similarly crumpled, and are at no very great depth where 
they underlie the sandstones here. Beneath these fractured and folded rocks, 
again, we come upon the gently imclined beds of a highly siliceous conglome- 
rate ; which beds vary, not at all in the nature of their constituents, but only 
in the amount of attrition to which they have been subjected ; they are alto- 
gether destitute throughout of vein, or seam, or cavity, within which a crystal- 
line or cleavable mineral had space or time to arrange its atoms. 

