542 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
yielding by its metamorphoses about one-third of its original bulk of carbonate 
of lime. 
But, in what are called the serpentinous marbles, quite a different state 
of matters exists. These marbles I know from one part of the bed in Tiree; 
from Loch Bhalumais, in Lewis; Rodal, in Harris; Dalnein, in Strathdon; 
Glen Elg; Glen Tilt, and some neighbouring localities. It is of these only I 
am entitled to speak. 
In all, the appearance is the same,—granular imbedded particles. of ser- 
pentine, from the size of a shot to that of a bean, sparsely sprinkled through- 
out a great mass of lime, in an amount which is altogether quite trifling. 
Unhesitatingly I say that these granulars are, one and all, pseudomorphs of 
pre-existent crystals of augite. That mineral may be seen unchanged and | 
changing in the Tiree marble. In Glen Elg and other localities the pseudo- 
morphic forms are so perfect that the crystalline form of the augite is indubi- 
table. 
These trifling specks ‘could never have been the origin of a lime stratum 
tens of feet in thickness. 
I would suggest the following explanation of the serpentinous formation of 
these pseudomorphs,—which had palpably pre-existed as augitic crystals 
imbedded in lime. 
It is well known that when silica or silicates are heated along with carbonate | 
of lime, there is, in the first instance, a disengagement of carbonic acid and a 
formation of silicate of lime; or, in the presence of other bases, of more complex 
silicates. Briscuor found this decomposition to take place unfailingly even at 
the temperature of boiling water. No great depth in the earth’s crust would 
suffice for the attainment of such a temperature. Thus at the point of contact 
of the lime with the including rock, and also with inclosed silicous minerals, 
would there be disengaged the very acid which we have already seen to be the 
active agent in serpentinous change ; and thus also would there be formed new 
silicates, such as Wollastonite, silicate of lime, and tremolite, which, in fact, are 
found associated with the pseudomorphs of the serpentinous marbles. 
In the outset I directed attention to the fact that serpentines were 
occasionally formed by the transmutation of such rocks as diallage and diorite | 
as a@ whole,—i.e., that the labradoric as well as the augitic ingredient had | 
suffered conversion. The analyses afford one if not two instances of such 
conversion, and I now instance another which may be a case in point. 
32. Beauty Hill, to the north of Aberdeen, is composed of serpentine, and 
in small quarrys on its north-east side it will be seen to be as distinctly bedded | 
a rock as any recognised sedimentary deposit. 
On its eastern slope some masses of gabbro protrude through the sward of | 
a field. This gabbro is, for the most part, unaltered. It consists of dark 

