PLUTOlfIC ROCKS OF GARABAL HILL AIsTD MEALL BREAC. 107 



The acid rocks are represented by porphyritic and non-porphyritic 

 biotite-granites. Between the two extremes we have numerous 

 intermediate varieties which may be grouped under such terms as 

 augite-diorite, diorite, tonalite (quartz-mica-diorite), and hornblende- 

 biotite-granite. 



The first rocks formed were peridotites ; then followed diorites, 

 toualites, and granites, in the order of increasing acidity. The most 

 acid rock known occurs as narrow veins iu the granite and tonalite. 

 It consists of felspar and quartz, and is almost entirely devoid of 

 ferro-magnesian constituents. 



Pacts similar to those referred to in this communication may, as 

 we have already stated, be observed in other parts of the Southern 

 Highlands. Thus, in the classic area of Glen Tilt there is a mass of 

 plutonic rock which is partly acid and partly basic in character. 

 As a rule the one type passes gradually into the other — granite 

 merges into diorite — but occasionally one rock veins the other, and 

 w^hen this takes place, the granite is always seen to be the younger 

 of the two. 



In the opening paragraph of this paper we referred to the dykes 

 and sills which abound in many parts of the Southern Highlands. 

 These, like the great plutonic masses, range in composition from 

 ^cid to basic ; so that we are compelled to include most of them 

 under the general and ill-defined term, lamprophyre. With the 

 exception of some of the felsitic dykes, which are seen to issue from 

 the granite, it is impossible to trace these lamprophyres to their 

 source ; but, looking at the general parallelism of the phenomena 

 presented by the dykes and the plutonic masses, we feel convinced 

 that they are both parts of the same series of events. 



We will now proceed to describe the physical, mineralogical, and 

 chemical characters of the principal rock-types. 



1. Specific Gravitt. 



The specific gravity of the principal rock-types is given in the 

 following list. It must of course be remembered that the specific 

 gravity of the peridotites has been reduced by serpen tinization. 



Serpentine, north-east of Loch Garabal, 2*76. 



Augite-serpentine rock, north-east of Loch Garabal, 3'32. 



Diallage-olivine-serpentine rock, north-east of Loch Garabal, 3*00. 



Enstatite-augite-hornblende-biotite rock, north-east of Loch Ga- 

 rabal, 3*15. 



Augite-diorite, north-east of Loch Garabal, 3*04. 



Diorite, west side of Ben Damhain, 2*96. 



Augite-diorite (much plagioclase), north-east of Loch Garabal, 

 2-81. 



Quartz-mica-diorite (tonalite), south-west of Ben Damhain, 2*76. 



Hornblende-granitite, junction with porphyritic granite, hill 

 south of Allt-na-Lairige, 1| mile from head of burn, 2*67. 



Porphyritic granite, hill south of Allt-na-Lairige, 1| mile from 

 head of burn, 2-62. 



Eurite, vein in tonalite, S.W. side of Ben Damhain, 2*59. 



