516 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 















the mixture may be said to be almost foliated; the felspar, however, is some- 
what granular. The rock is sometimes of so minute a structure that its grains 
are little larger than mustard seed. A hand specimen would be pronounced 
a fine-grained gneiss. CUNNINGHAME speaks of this as a slaty variety, but does 
not seem to have observed the replacement of hornblende by mica. A rock 
having the same components, but of much coarser structure, is to be seen on 
the south side of Craig Burn, in Auchindoir ; and, in a dyke at New Merdrum 
near Rhynie, there are two large masses of the same rock, in which the crystals 
of both the bronzy mica and of the labradorite—here grey—are one to two 
inches in size. 
The rock from these last-two localities contains nothing but labradorite and 
Biotite, if we except a very occasional speck of iserme or magnetite: it seems a 
straining of nomenclature to apply the term dzorite to such.a compound, but it 
must be remembered that the gradual interchange of augite for hornblende, 
and then of Biotite for augite, can be traced in an altogether unmistakable 
manner. It has to be stated that, except at Cowhythe Head, the rock is not 
in the slightest degree fissile. 
CUNNINGHAME clearly defines what is indubitably a variety intermediate 
between the two last varieties. In this, true hypersthene (¢.e., Paulite), augite, 
and bronzy mica take the place of hornblende. This rock, of which a fine- 
grained variety may be seen 7 situ on the west side of the Bay of Durn, 
would appear to extend up the country, to re-appear on the west side of Craig 
Buirroch, and near Retannoch. In these localities the constituents of the 
rock are best seen in exfiltration veins. These veins carry crystals of a yel- 
lewish-brown, translucent augite (analysis, page 466), bronzy Paulite, menacca- 
nite, pyrite, pyrrhotite, and rarely a bronzy mica. 
The rock in which these veins occur is a fine-grained mixture of apparel 
identical ingredients; and also with no hornblende. As one—the platey 
cleavage of the augite—is dominant, this rock may be called a hyperitic gabbro. 
But, if not diorite, what name should be given to the binary compound of 
labradorite and Biotite, I know not. 
Two questions at once suggest themselves in connection with these varieties. 
The first is—Are these not indications of the passage of the rock into other 
rocks @ 
Let us see what CUNNINGHAME says on this point. At page 488 of the | 
“Transactions” he says—“ At Portsoy the syenite is connected with hornblende | 
rock and hornblende slate, into which it passes in the most gradual and insensible 
manner :” at page 489—“ We assert that there is a gradation from the one into 
the other as perfect as may frequently be found to take place between gneiss | — 
and granite, and to which it seems to be analogous.” 
In the same page he also describes a locality near Boyndie, where there is a|_ 
