P. MACNAIR ON ROCKS OF HIGHLAND PERTHSHIRE. 187 
near the head of Loch Lomond and on the confines of Perthshire 
and Dumbartonshire. The rocks seen in this plutonic complex vary 
from the most acid to the most basic type, and include granite, 
tonalite, and diorite, which pass into such highly basic rocks as 
wehlrite, picrite, and serpentine. At some places it is possible to 
draw a distinct line of demarcation between the diorite and the 
tonalite, but at others the transition from one rock to another is so 
gradual as to make any such sharp distinction impossible. On the 
whole, these authors say;—“The south-eastern portion of the plutonic 
belt is more basic than the north-western. At certain points along 
the edge of the boss the diorite and tonalite are seen to invade the 
surrounding schists, sending veins and strings into the latter, and in 
some cases completely isolating blocks of the schist.” And again;— 
“ The area occupied by granite is largely in excess of that occupied 
by diorite. Taking the whole plutonic area as amounting to 12 
or 13 square miles (say 12J), 10 of these are occupied by granite 
and only 2J by diorite. - Diorite, therefore, only forms i of the 
total area. The area occupied by ultra basic rock is exceedingly 
small, amounting at the most to J square mile.” 
The hypothesis put forward by Messrs. Dakyns and Teall to 
account for the association together in one boss of such extreme 
types of igneous rock is that they were all originally intruded as one 
magma, the more basic part having cooled first around the margin 
into peridotites, these being succeeded by the diorites and tonalites, 
and finally the granite as the most acid type having cooled last, 
and nearest the centre of the mass. They also state that similar 
phenomena are to be seen in other parts of the southern Highlands, 
and they instance a case in Glen Tilt where a mass of plutonic rock, 
partly acid and partly basic, may be seen. As a rule, they say, the 
one rock gradually passes into the other, but at some points the 
granite is seen to vein the diorite, which shows the acid rock to be 
the younger of the two. This is identically the same phenomenon as 
that which we have just instanced as occurring at Tomnadashan, 
on Loch Tay. 
Like the altered basic rocks and clastic schists, these later 
intrusive rocks are shrouded in a mystery heavy and difficult 
to lift. What is their age, and can they be connected with any 
period of volcanism, are among the more important questions that 
still await solution. It will be remembered that Prof. Judd, in his 
second paper upon the “ Secondary Rocks of Scotland,” ^ published 
1874, brought forward the view that these plutonic rocks were but 
the roots of volcanoes laid bare through extensive denudation, and 
that he attempted to connect them with the volcanic products of 
Old Red Sandstone age. In this paper Prof. Judd also institutes 
