OF THE TRAP-ROCKS OF SCOTLAND. 641 
We there meet with lava streams and thick bands of interstratified ash and con- 
glomerate, forming sometimes long chains of hills, and showing that the igneous 
forces not only existed below, as they did in the south, but that they continued 
to operate at the surface during a long geological period. The Sidlaw Hills, and 
the chain of heights cut through by the Tay at Perth, are illustrations of the 
character and extent of these ancient volcanic eruptions. 
There is one characteristic section, hitherto (so far as I am aware) undescribed, 
laid open by the railway, about a mile south of Dunkeld. It shows the great 
difference lithologically between the felspathic rocks of the Highland region and 
those ejected contemporaneously with the deposition of the Lower Old Red Sand- 
stone of Perthshire. During the greater part of last summer I was engaged, along 
with Sir R. I. Murcuison, examining the crystalline metamorphic rocks of the 
Highlands, and devoted particular care to the discrimination of the various granites, 
granulites, syenites, porphyries, and felstones, which occur in that excessively 
altered region. Returning southward by Dunkeld, we visited the railway cutting 
there, beginning near the south end and walking northwards to the town. The 
first rock which met us was a pebbly conglomerate, which I was surprised to 
find consisted almost wholly of felspathic fragments. A scrutiny of several 
yards of the rock was sufficient to satisfy me that the fragments in question 
were not derived from the Grampians. They differed altogether from the com- 
pact crystalline quartziferous felstones of the metamorphic region, and resembled | 
the interbedded rocks of other districts with which I was familiar. As we pro- 
ceeded towards Dunkeld, lower beds of the series came successively into view, 
and the component fragments became gradually coarser, until they occurred as 
large as two feet, or even more, in diameter. They were dark, vesicular, slaggy- 
like masses, often quite angular, and were accompanied with only rare and small 
fragments of slate. At last we came to a band of dark cellular felstone, identical 
with some of the larger blocks just referred to. It was clearly a contemporaneous 
ejection, and showed the vesicular scoriaceous aspect, so common on the upper 
surface of a sheet of interbedded trap-rock. Below this band of felstone another, 
but less felspathic, series of conglomerate beds succeeded, which rested finally on 
the broken and twisted edges of the clay-slate. 
From the railway we could see that this igneous series stretched towards the 
north-east as a line of rounded eminences. It seems probable, also, that not a 
few of the trappean patches inserted on the maps across the area of Perth and 
Forfar may have had a similar origin, and may belong to the same geological 
period. It becomes important also to inquire whether, as the conglomerate of 
Dunkeld is undoubtedly connected with the operation of volcanic forces, other 
portions of the great conglomerate series which flanks the Grampians may not 
have arisen, to some extent at least, from the same source. 
There is thus clear proof of the existence of active volcanic force in Perthshire, 
VOL. XXII. PART III. 8D 
