164 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
of these rocks are always more or less coincident with the main lines 
of plication in the Highlands. Thus on a cursory view the hypothesis 
advanced originally by De la Beche and Jukes, and latterly by Bonney, 
McMahon and others, that these hornblende schists were the meta¬ 
morphosed equivalents of volcanic ashbeds, and that the banding 
and similar structures were referable to deposition in water, might 
seem not to be without some support. But we believe that when the 
whole evidence is considered it will be found to be in favour of the 
view that most of the hornblende schists found in the southern 
Grampians are of igneous origin, and it is from this standpoint we 
would seek to explain all the phenomena seen at Balhoulan Quarry. 
Though at Balhoulan we have not been able to trace the horn¬ 
blende schists to any distance beyond the quarry, yet we believe from 
the general appearance oi the rock that, like similar sills seen in the 
neighbourhood of Loch Tay and Loch Earn, could it be traced for 
any distance it would be seen to transgress the clastic rocks, breaking 
across from one horizon to another. The fact that the division 
planes of the rock are lines of shear makes it exceedingly difficult to 
trace the above phenomena in a limited section such as that exposed 
in Balhoulan Quarry, the hornblende schists appearing deceptively to 
be intercalated between the bedding planes of the clastic schists. 
Again, the more highly mineralised condition of the clastic rocks 
along the line of contact with the hornblende schists points most 
conclusively to the intrusive origin of the latter. Along the lines of 
contact at Balhoulan, between the clastic rocks and the hornblende 
schists, an enlarged metamorphism of the former can be distinctly 
traced, the bands of limestone and grit being much harder and more 
mineralised in comparison with those that have only suffered from 
regional metamorphism. The occurrence of masses of fragmental 
rocks included in the hornblende schists of Balhoulan Quarry is in 
itself sufficient to place their igneous origin beyond dispute. We 
think the evidence we have adduced for the fragmental origin of the 
banding in these rocks is conclusive, and directly points to the fact 
that the hornblende schist must have originally been intruded into 
these rocks in a molten state. 
Let us now consider, in conclusion, the manner in which the 
banded structures in these schists have been produced. As we have 
already seen, the mode of occurrence and mineral structure of these 
hornblende schists entirely precludes the possibility of their aqueo- 
volcanic origin, and is strongly in favour of their intrusive igneous 
nature, so that the original theories of Bonney and M‘Mahon regard¬ 
ing the banding of the Lizard schists cannot be applied to the horn¬ 
blende schists of the southern Grampians, nor to the section seen at 
Balhoulan Quarry. 
