THE GEOLOGY OF SOUTH-EASTERN KINCARDINESHIRE. 937 
lavas, and tuffs, and, in particular, the recognition of a well-marked succession in 
the lavas, has aided materially in elucidating the structure of the area occupied by 
the Lower Old Red Sandstone. 
The rapid variation in the character of the rocks in any given horizon makes it 
almost impossible to draw a sharp line of demarcation between one zone and another. 
For convenience in description, however, I have divided the Lower Old Red Sandstone 
series into five groups, arranged as under in descending order :— 
The Strathmore Group. 
. The Garvock fe 
The Arbuthnott _,, 
The Crawton * 
- The Dunnottar __,, 
rFWOOUOR 
The approximate thickness of each group is as follows :— 
Maximum Minimum 
feet. feet. 
é : : : : . 6900 } 
Dunnottar Group 5 | Loan 
Crawton y : : ‘ : . 1600 
Arbuthnott_,, ; : : ; . 5000 3000 
Garvock 26 ; ; : ; . 4000 3800 
Strathmore _,, : ‘ : ; .- 1500+ 
The marked variation in the thickness of the three lowest groups is due in large 
measure to the varying development of the intercalated lava zones, and to the rapid 
lateral variation of the associated volcanic conglomerates. 
The above figures confirm Sir ARCHIBALD GEIKIE’s* estimate of 20,000 feet as the 
maximum thickness of the Lower Old Red Sandstone of Lake Caledonia, included in 
which are the “‘ Stonehaven Beds” (2760 feet), regarded in this paper as Downtonian. 
A. The Dunnottar Group. 
The Dunnottar group includes the part of the Lower Old Red Sandstone which lies 
between the base of the series and the top of the Tremuda Bay lavas. A brief description 
of the coast section from Stonehaven to Thornyhive Bay will perhaps serve to indicate 
the general aspect of this portion of the Old Red succession. Its chief characteristic is 
a magnificent development of coarse conglomerates with intercalated thin brown sand- 
stones. In the conglomerates well-rounded quartzites are always the most conspicuous 
constituent. They are accompanied by other ‘‘ Highland” rocks and by a varying 
proportion of granites of “newer granite” type, quartz porphyries, rhyolites, and acid 
andesites. Boulders derived from the Jasper and Green-rock series are present, as a rule, 
* Text-book of Geology, vol. ii. p. 1008, 1903. 
