P. MACNAIR ON ROCKS OF HIGHLAND PERTHSHIRE. 185 
basic rocks the hornblende generally presents the characteristic long 
needle-like forms, or occurs as grains or irregular masses in the horn¬ 
blende schist. The amount of metamorphism which the sill has 
undergone seems to vary considerably at different points. Thus at 
Killin it has passed into a typical hornblende schist, having at this 
point evidently undergone the maximum of change. On Meal-na- 
Creig and in Glen Quaich the structural change has not been so 
great, as at these points it does not exhibit the schistose structure, 
though the original component minerals of the basic rock show 
evidence of having undergone the usual changes consequent upon 
being subjected to the action of dynamo-metamorphism. 
Many obscure and difficult problems still await solution in con¬ 
nection with these altered basic rocks. It is probable that when 
they have been subjected to a detailed examination they will be 
found to consist of different types of basic lava. Again, the con¬ 
ditions under which they were erupted is still a matter surrounded 
by much mystery; for while they generally show evidence of having 
been intruded as sills amongst the clastic rocks, as in the case of the 
mass underlying the Loch Tay Limestone, it is probable, on the 
other hand, that some of them may have been erupted at the 
surface as lava flows. This view is borne out by the presence of 
green chlorite-schists, which have evidently originally been volcanic 
ashbeds. It is often, however, very difficult to determine correctly 
what the original relationship of many of these masses of basic 
rock may have been. So completely have they been rolled out 
between the shearing planes of the mica schists that their original 
lines of contact with the clastic schists have become more or less 
obliterated. 
We proceed now, in conclusion, to a brief consideration of some 
of the more recent advances that have been made in the study of 
those later intrusive rocks, which are found so well developed in 
Highland Perthshire, as in the Moor of Rannoch, the valley of 
Loch Tay, Glen Lednock, and elsewhere. Along the whole of the 
southern Grampians we find these later intrusive rocks more or less 
developed, as for instance the extensive tracks of intrusive granites 
in Aberdeen and Kincardine in the east, and those around the head 
of Loch Etive and Loch Lomond in the west. 
Undoubtedly the most striking feature of these later intrusive 
rocks, and one which has long been known, is the association 
together in one plutonic mass of some of the members of the two 
extreme types of acid and basic igneous rocks. Thus, at Cairn 
Chois, in Glen Lednock, we have a granite and a diorite intimately 
connected together; while at Tomnadashan, on Loch Tay, a similar 
relationship is seen to exist between a granitite and a micaMiorite. 
