PROCEEDINGS OE THE PERTHSHIRE SOCIETY OE NATURAL SCIENCE. 
19 
of the less metamorphosed portions of the Silurian strata 
of Perthshire. 
I come now to say a few words about the Old Red 
Sandstone strata. These strata represent both upper and 
lower divisions of the Old Red Sandstone system. The 
lower division is by far the more abundantly devel¬ 
oped of the two,—the upper series appearing only in 
the Earn valley near Bridge of Earn, in the valley of the 
Tay between Glencarse and Errol, and in the neighbour¬ 
hood of Inchture, while the lower series forms by far the 
larger part of the lowlands of the county, the Sidlaw Hills 
being built up of the same. The lower series abuts against 
the foot-hills of the Grampians—a wavy line drawn from 
Aberfoyle north-east by Glenartney, Buchanty, and Dun- 
keld to the Reekie Linn near Alyth, forming the boun¬ 
dary between the Silurian on the one hand and the Old 
Red Sandstone on the other. 
If we traverse the Old Red Sandstone from north-west 
to south-east, we shall find that the strata are arranged 
in a very definite manner. Along the flanks of the 
Grampians the beds are either vertical or highly-inclined 
towards the south-east, but as we follow them in this 
direction the angle of dip rapidly diminishes until about 
the middle region of Strathmore the beds become ap¬ 
proximately horizontal. Thereafter, as we approach the 
Sidlaws, they begin to rise up, as it were, in the oppo¬ 
site direction,—dipping to north-west,—a dip which con¬ 
tinues until we have passed the crest of the Sidlaws, when 
they again roll over and dip towards the south - east. 
Strathmore thus lies in a great synclinal trough, while the 
Carse of Gowrie runs along the crest of an anticlinal 
arch. 
The lower beds of the series are best seen along the 
flanks of the Grampians, where they are largely made 
up of conglomerates, along with which occur bedded 
masses of igneous rock. These rocks are well exposed 
in the Isla to the north of Alyth, in the Ericht Water, 
near Birnam, at Buchanty, in the neighbourhood of 
Monzie and Ochtertyre, in Glenartney, and at Cal¬ 
lander. Dipping underneath the more gently - inclined 
strata in Strathmore, the lower series rises up again to 
form the Sidlaws and the Ochils. In the Sidlaws the 
strata are largely composed of various kind of igneous 
rocks, chiefly porphyrites, with tuffaceous beds and ashy 
conglomerates. As we follow the series in the Sidlaws 
north-east into Forfarshire, sandstones, flagstones, and 
tilestones become more and more abundant, until they 
come to form the major portion of the hilly tracts. In 
Perthshire, however, the Sidlaws are formed essentially 
of ancient lava-flows. 
The conglomerates of the lower series form an in¬ 
teresting study, and might well engage the attention of 
local geologists. It will be found that the stones are 
composed of many varieties, some of them derived from 
the Silurians and some from the Old Red Sandstone 
itself. It is clear that at the time of their formation 
the flanks of the Grampians formed an old shore-line, 
along which the conglomerates were accumulated as gravel 
and shingle banks. During the time of their formation 
local volcanoes poured out lavas upon the bed of the inland 
sea, and these, broken up by the action of the denuding 
forces, contributed no small proportion of' the water-worn 
stones that now go to make up the conglomerates. It 
would be extremely interesting to form a collection of the 
various kinds of stone which occur in these conglomerates, 
and to trace out the parent-rocks from which they have 
been derived. It will be found that a large number have 
been brought down from the Highland hills, probably by 
rivers and torrents. I have, indeed, been led to believe 
that by far the larger proportion of the stones owe their 
origin to stream and torrential action; and the peculiar 
distribution of the conglomerates, and the very large size 
attained by the stones in some places, together with their 
frequently angular and subangular forms, lead one to sus¬ 
pect that at the time of this accumulation glaciers may 
have existed in the Highland glens. In the Sidlaw Hills 
the conglomerates are not so strongly developed; but here 
and there, as in the hills above Kinfauns Church, they 
attain a considerable thickness. 
The porphyrites, with their accompanying ashy beds, 
likewise afford a wide field for study. The slaggy char¬ 
acter of the upper and lower portions of the ancient 
lava-flows is often beautifully exhibited, especially in 
Moncreiffe Hill, Callerfountain Hill, Dunsinane Hill, and 
many other localities. 
As a rule, the sandstones of the lower series which extend 
throughout Strathmore show no fossil organic remains. 
Now and again, however, faint and imperfect traces of 
plants may be detected. It is worth noting that these 
sandstones often contain sporadic water-worn stones, and 
now and again a local bed of conglomerate makes its appear¬ 
ance. The whole series appears to have been deposited in 
a wide inland sea, upon the bottom of which the volcanic 
forces erupted bed after bed of molten rock. Upon the 
highly-denuded surface of the lower series rests the upper 
group of red sandstones which are seen in the neighbour¬ 
hood of Bridge of Earn, at Clashbennie, and Inchture. It 
is evident that before they were laid down the lower series 
had been thrown into folds, faulted, and very much worn 
and denuded. It is probable, indeed, that the lower valley 
