20 
PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
of the Tay had actually been already sketched out. At 
Clashbennie the sandstones have yielded remains of fishes 
and crustaceans. 
I have mentioned the Carboniferous as forming part 
of the geological series of Perthshire. The rock of this 
age, however, appear to be restricted to the small de¬ 
tached part of the county that adjoins Clackmannan and 
Stirling shires on the Frith of Forth. 
Passing over all the strata of the Mesozoic or Secondary, 
of which we have no representatives, we reach the Tertiary 
or Cainozoic rocks. Of these the only relics we have are the 
great east-and-west dykes of basalt-rock, some excellent 
examples of which occur in our own neighbourhood. Two 
of the most marked begin in Moncreiffe Hill, and can be 
followed west by Aberdalgie and Gask to beyond 
Drummond Castle. They form most conspicuous feat¬ 
ures in the landscape, often rising up as a bold 
ridge above the general level of the sandstone-strata, 
which they intersect. They are the youngest of all the 
igneous rocks of our county, and are believed to belong to 
the Miocene or Middle Tertiary period. 
As the rocks of these dykes form most durable cause¬ 
way - stones and are the best of road - metal, they are 
quarried in many places, and thus admirable sections 
are available for study. It is particularly instructive 
to observe the alteration which has been produced by 
the igneous rock upon the sandstones which it cuts 
through. These latter are hardened, baked, and 
cracked at the line of junction—clearly showing the 
effect of great heat. The basalt-rock itself also tells its own 
tale in its peculiar structure. As a rule, the rock of the 
dyke is considerably coarser-grained in the centre than to¬ 
wards the side. In the centre it is generally coarsely 
crystalline and not unfrequently vesicular, while towards 
the sides it becomes finer in the grain, until at the point of 
contact it is often quite compact. These changes show the 
manner of cooling. It is evident that the sides of the 
molten mass in contact with the stratified rocks would 
cool most rapidly,—and the crystalline structure under 
such circumstances would not be so distinctly developed. 
Towards the centre the molten matter would consolidate 
more slowly, and thus the individual crystals would be 
better enabled to accrete, while the imprisoned gases and 
steam would give rise to the small cavities,—which resemble 
the similar cavities in pumice and common furnace-slag. 
The cavities, I may add, are now usually filled up with 
some mineral which has subsequently been deposited in 
them by the water which is always percolating through the 
pores and interstices of a rock. 
I come now to the most recent of our formations, 
namely, the Post-Tertiary and Recent deposits. These 
are almost everywhere present, and yield in interest to 
none of the older rocks, which they overlie and partially 
conceal. 
The oldest of the Post-Tertiary accumulations is the 
well-known till or boulder-clay, which in the districts 
outside of the Grampians is usually red, while in 
the Highland glens the prevailing colour is a pale grey¬ 
ish brown, inclining in some places to yellow and blue. 
In Strathmore and the low country generally it is as a rule 
a tough tenacious clay, destitute of bedding, and abund¬ 
antly charged with subangular and angular boulders and 
stones,—many of which are smoothed and scratched. 
This till is the bottom-moraine of an old glacier-mass, and 
the direction in which the ice flowed can be determined by 
noting the course followed by the stones in the till. These 
stones have been rolled forward underneath the ice. In 
our own neighbourhood we find that they have all come 
from the north-west;—some from the Highlands, others 
from Strathmore. The prevailing red colour is due to the 
fact that a great proportion of the mass consists of the de¬ 
graded and pounded materials derived from the glacial ab¬ 
rasion of the red sandstones and marls of Strathmore. 
When we enter the Highlands we find that the till is com¬ 
posed exclusively of Highland rocks, and that the colour is 
no longer red, but, as I have said, pale brown. 
When the till is dug up, and the rock upon which 
it rests has been exposed, we may note that the sur¬ 
face of the latter is often roughly smoothed, polished, 
and scratched. And if we take the compass - bearing 
of those scratches, we find that they coincide in direc¬ 
tion with that followed by the stones in the till. 
These scratched rocks are evidently the work of the 
old glacier, which, by dragging over its bed stones, 
sand, and clay, smooth, scratched, and polished its bed 
in the same manner as the modern glaciers of Norway and 
the Alps. It is not only underneath the till that these 
scratched rock-surfaces may be detected. We note them 
on the slopes, and even the tops, of some of the hills. 
I have seen them near the very top of Birnam Hill and 
many other hills in its neighbourhood, and they may be 
detected every here and there upon the very crests of the 
Sidlaws and Ochils. They prove that all this region was 
formerly buried underneath ice which overflowed from the 
Highlands, sweeping across hills up to a height of 3000 
feet, and pressing out in a general south-east direction. 
Even when the scratches are absent, a practised eye will 
note how the rocks give every evidence of having been 
ground and abraded by some force pressing against them 
from north-west to south-east. 
