B. K. N. Wyllie & A. Scott— Plutonics of Garahal Hill. 539 



which might almost be called aphanitic in structure, are found in 

 contact with the schist, while elsewhere the crystals of hornblende 

 average 3 cm. in width, such coarse rocks generally occurring some 

 distance from the margins. Another significant feature is the dyke or 

 boss-like form in which the hornblendites occur. ^ It seems to us, 

 therefore, that they may be regarded as the roots or cores of giant 

 apophyses of the diorite projecting into the schist roof and now laid 

 bare by denudation. The fine-grained margins would crystallize 

 quickly and form an impermeable envelope, which would inhibit the 

 escape of water and other mineralizers and favour the formation of 

 hornblende. Hence these may be compared to pegmatites on a large 

 scale, resembling the ' peridotite-pegmatites ' of Loch Garabal in the 

 mode of formation, though the latter are on a much smaller scale. 

 The abundance of hornblende also favours the pegmatite theory, as 

 mineralizers such as water are essential for its formation. 



II. 



IV. 



II. 



IV. 



Fig. 3. Variation diagram of rocks of loc. v. Note : Quartz and biotite curves 

 are drawn with scale of ordinates doubled. 



The mineralogical variation in the rocks of this series is most easily 

 shown by means of a diagram. A number of specimens were taken 

 at approximatelj^ uniform distances from the tonalite and their 

 mineralogical composition calculated by the Eosiwal method.^ The 

 hornblende increases from 7 to 84 per cent, while the felspar shows 

 a complementary decrease from 70 to 5 per cent. Quartz, which is 

 never very abundant, decreases to a minimum before the coarsest rock 



•• Hornblendites with brown amphibole have been described by Wright and 

 Bailey in Colonsay, forming the margin of a syenite intrusion: Geology of 

 Colonsay (Mem. Geol. Surv. Scotland), 1911, p. 29. 



'■^ No. 1 is an acid diorite from Meall Breac, while the others are from the 

 Garabal Burn. 



