1 68 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
of these great intrusions. It will be seen that here also many of the 
more recent views regarding the structure and origin of these plutonic 
rocks had already been anticipated in some form or other, and more 
especially that which now regards the two extreme types of acid and 
basic rocks found so intimately associated with one another as having 
been differentiated out from the same magma. It will be unnecessary 
for us to refer to any of the material written on the structure of the 
Perthshire Highlands during the period preceding the publication of 
Murchison, Geikie, Harkness, and Nicol’s papers of 1860-63, for 
with the contributions of these authors the real study of the Highland 
problem, as we now know it, may be said to have commenced. 
In their papers on the altered rocks of the Islands and north-west 
and central Highlands of Scotland, published i860, Murchison and 
Geikie’- describe certain sections along the margin of the southern 
Grampians, with reference to the supposed succession of the 
rocks in the north-west. Briefly put, it was believed that the 
southern and central Highlands were overlaid by a great series of 
quartzites, limestones, and schists, the representatives of those found 
in the north-west of Sutherlandshire, and that they always occupied 
the same relative positions to one another—namely, the quartzites 
at the base, these followed by the limestones, and the whole overlaid 
by the schists. Let us now glance at the manner in which these 
authors read this supposed succession into the rocks of Highland 
Perthshire. In their description of the area between Tyndrum and 
the head of Loch Tay, Murchison and Geikie refer the higher 
reaches of Glen Dochart to the so-called upper schistose group, while 
the quartzose group is ushered in near the bottom of the Glen along 
with the limestones. A similar structure was supposed to exist in 
the case of Loch Tay and Ben Lawers, the limestone exposed along 
the base of the latter mountain and both sides of Loch Tay marking 
the division line between the upper schistose series and the lower 
quartzose group. From the following sentences we would infer that 
the authors did not look upon the Loch Earn and Loch Tay 
limestone as being upon the same horizon, and it also appears that they 
did not quite understand the relationship of the limestone band 
towards the south. “ We are not aware,” they say, “ how far the 
limestones stretch towards the south-west. Those of Lochearnhead 
appear to belong to another arch. Towards the north-east of Loch 
Tay they are soon lost, but reappear in the valley of the Tummel at 
Pitlochry.” 
In their description of a cross section from Loch Tay to Loch 
Rannoch, a succession similar to the foregoing was there supposed 
to be traceable, the schistose rocks around Kenmore being refer¬ 
able to their upper group, while the quartzites of Schiehallion were 
