
PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 477 
from chlorite, however, must be held to exclude it from the former; and we are 
thus brought back to reassign it to the latter—a hyperite in which augite takes 
the place of hypersthene, and so one under the category of which the Rum 
rock also immediately falls.* 
The augite of Rum is of at least two well-marked varieties. The first 
and most common may be picked up in loose rough crystals, which strew 
the gravelly banks under the pyramidal second heave of Halival; they lie 
glancing in the sun in quantities. This variety is dull green when fresh, but 
always opaque ; they weather brown, but never bronzy (iron-brown, JAMESON) ; 
they resemble the ordinary variety of the Cuchullins. 
The second variety is transparent or translucent. It was first noticed by 
JAMESON, who considered it to be pitchstone. He thus describes it :—“ Crystal- 
lised pitchstone, top of Halival, Rume. It is of an olive or dark leek-green 
colour, and has the usual lustre, transparency, and hardness. The crystals are 
immersed in a rock formed of felspar, with a few scales of tombac-brown 
coloured mica. They are from the tenth to the half of an inch long. I could 
not discover the form of the crystals, on account of their being much broken. 
It is well deserving the attention of those who visit Rume.” . 
Crystals of considerably larger dimensions than here noted are to be found. 
Bright yellow olivine is another associate, and the labradorite crystals are finely 
striated, and the most pellucid in Scotland. This transparency is also the 
marked feature of the augite; and, taken in conjunction with its vitreous 
lustre, and the fact that it does not cleave, but yields conchoidal fractures, quite 
accounts for JAMEsoN’s having taken it for a variety of pitchstone. It might 
also quite excusably be taken for olivine, to which it bears a much greater 
resemblance than does the associated yellow olivine itself. 
* While correcting the proofs of this paper, I have, for the first time, had the pleasure of perusing 
Professor Jupn’s classical research on the rocks of Mull. In this I find that he, throughout, unhesi- 
tatingly calls the Rum rock gabbro. . Now—whatever rock Von Buch originally applied the name to,— 
the term gabbro is now, I believe, universally attached to a combination of labradorite with diallage. 
IT went round the whole shore of Rum, traversed the island in three directions, ascended seven of its 
highest hills, skirted the flanks of these, and yet, not only did I never see a particle of diallage in the 
island, but I never saw a bit of the augite which even tended in features to that modification. I need 
hardly, therefore, say that I saw no gabbro. It may be held that this is mere fighting for a word; but 
if is not so, it is fighting for precision in language. If gabbro is to be extended or loosened in its 
application so as to include an ordinary augitic rock, I have nothing to say, further than to show that 
it would be most unadvisable to do so. But till a consensus of geologists resolves to do so, it must 
tend to endless confusion, that the author of so masterly a research as that referred to,—speaking almost 
ez cathedra,—applies the term to a rock so absolutely different in appearance as the present is from any 
typical gabbro. I am the more sensitive as regards writing by the book in the present case, because, so 
far as my experience goes, the gabbros of Scotland—that is, the rocks carrying thin, foliated lustrous 
dugite, and they are very typical—are all metamorphic. Be this as it may, it is not advisable that the 
name to be adopted for a class of rocks should be one which in the past has designated a variety, 
the marked feature of which is, that it contains a peculiar modification of that substance which is the 
chief constituent of all the members of that class. We might, it is true, designate that variety by the 
term diallagite ; but let us understand each other, as a necessary preliminary to our entertaining the 
hope that others will understand us. 
VOL. XXVIII. PART II. 6A 
