642 MR ARCHIBALD GEIKIE ON THE CHRONOLOGY 
and in the Sidlaws of Forfar, during the accumulation of the Lower Old Red 
Sandstone. To the same period, I am at present disposed to refer part of the range 
of the Ochils. The north-eastern prolongation of these hills is undoubtedly of 
the age, not of the lower, but of the Upper Old Red Sandstone, since the felstones 
and conglomeritic bands pass upward, and become intercalated with the yellow 
sandstones of Dura Den. But to the south-west, beyond Kinross, there appears 
to be unconformity of the Mountain Limestone on the trappean series, which is 
highly inclined, and ought probably to be connected with the Lower Old Red 
Sandstone of the valley to the north. But much careful research is still needed 
before the geological structure and age of this group of hills are understood. 
The Campsie Hills, notwithstanding the able descriptions of them by Colonel 
Imriz and others, are not yet brought into thorough chronological order. From 
the paper and sections by Mr Youne of the Hunterian Museum, Glasgow,* and 
from the information he has given me personally, I infer that these hills are on 
the whole contemporaneous with the north-east end of the Cchils; that is, of the 
age of the Upper Old Red Sandstone. 
Passing still south-west, parallel to the boundary of the metamorphic region 
of the Highlands, other chains of hills are met with, consisting, like those already 
noticed, of igneous rocks. They stretch to the Clyde at Dumbarton, whence they 
spread southwards into a high undulating tract of country, forming the main ridge 
or water-shed of the counties of Renfrew, and part of Lanark and Ayrshire. 
From observations made at various parts of this range of hills, I regard the whole 
provisionally as belonging to the Old Red Sandstone. As in the case of the Ochil 
and Campsie Hills, the great Lower Carboniferous series of the Lothians is here 
absent, and we find the Mountain Limestone group frmging these old volcanic 
banks. 
No better section of part of these trap-rocks exists than that which has been 
laid open by the waves of the Atlantic, along the coast of Ayrshire, from the 
Heads of Ayr southward to beyond Turnberry Point. The true interbedded 
character of the masses is there abundantly evident. Beds of compact felstone, 
sometimes strongly amygdaloidal, are intercalated among strata of ashy conglo- 
merate, red sandstone, and marl,—the trappean ridges running out to sea as 
reefs, while the softer beds have been worn into sheltered bays, that look across 
the blue firth to the grey mountains of Arran. 
Much still remains to be done in the region between the Ayrshire coast and 
the southern end of the Pentland Hills. In the course ofa rapid excursion made 
in bad weather, towards the close of last autumn, I saw, at Corsancon Hill, some 
felspathic conglomerate of Lower Old Red Sandstone age (hitherto marked as 
trap), and have little doubt that part of the felstone ridge which bounds the 
* Transactions of the Geological Society of Glasgow, Part I., 1860. 


