178 transactions—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
^ It will be recollected that at the outset of this paper, when dealing 
with NicoFs theory of the structure of the southern Grampians and 
the relationship of the clay-slates to the mica-schists, we showed that 
Nicol believed that along the margin of Highland Perthshire the 
clay-slates occupied an abnormal position. His theory was that they 
did not really underlie the mica-schists, but that the latter are the 
older rocks, which have been pushed and faulted over the clay-slates. 
Now we find Sir A. Geikie, in his later Government Reports,® reverting 
to this old theory of Nicol’s, and receding in part from the position taken 
up m his Presidential Address of 1891, and the succession there 
published. We quote from his Report of last year (1896);_“ In my 
Report for 1893 reference was made to a belt of rock, not improbably 
of Lower Silurian age, interposed along the Highland border between 
the schists on the one hand and the fault that brings down the Old 
Red Sandstone on the other. Mr. Clough was instructed to resume 
the examination of this belt between Aberfoyle and Loch Lomond. 
Notwithstanding his detailed mapping, to which reference has already 
been made, we are still unable to form a definite conclusion as to 
the structure of this difficult piece of ground. Mr. Clough is on the 
whole inclined to believe that between the black shales and cherts, 
which may be Lower Silurian, and Aberfoyle grits which lie imme¬ 
diately to the north of them, and form apparently a continuous 
portion of the Highland rocks, there are indications of a discordance 
or stratigraphical break.” It is, however, admitted in this Report that 
a great difficulty arises in the attempt to fix any line of demarcation 
between these supposed younger rocks and the grits and phyllites 
presumably belonging to the Highland rocks. It is also admitted 
that these grits are no more altered than the shales and cherts of the 
supposed younger rocks; while, further, it is often observed that the 
shales, phyllites, cherts, and grits are so much interbedded as to point 
to the conclusion that they form one stratigraphical group. Again, 
to quote from the Report: “ The obscurity of this boundary line has 
suggested ffie inquiry whether the Aberfoyle grits should not be 
grouped with the black shales and cherts as probably also of Lower 
Silurian age. But this explanation would only shift the difficulty 
farther north, for we would still have to find somewhere a southern 
line of demarcation for the schistose rocks of the Highlands. 
Certainly, so far as detailed investigation has yet gone, no such line 
is to be found between the Trossachs and Aberfoyle.” It seems to 
us that these views are nothing more than a resuscitation of the old 
theory advanced by Nicol many years ago to account for the position 
of these less altered rocks along the margin of the Grampians, 
In the Go\ernment Report for 1897 Sir A. Geikie states that J\Ir. 
c to distin^jUish a series of unaltered rocks 
