302 
VOLCANOES OF THE LOWER OLD RED SANDSTONE 
BOOK V 
surpasses in thickness any otiier conglomerate series in the Lower Old Eed 
Sandstone of Britain. Thi’oughout the enormous depth of sedimentary 
material, the conglomerates are well-bedded, consisting of a dull green paste, 
composed in large degree of comminuted andesitic debris, and inter- 
stratified with green felspathic sandstones. They are often remarkably 
coarse, the pebbles sometimes measuring three feet in length. Interposed 
among them are some ten or twelve bands, probably often single outflows 
of andesite, sometimes compact and porphyritic, at other times highly 
amygdaloidal. Such is the succession of rocks for many miles along the 
shore ; and as the inclination varies from a little north of west to west, or even 
west by south, the observer gradually passes over a thickness of rather more 
than 2000 feet from the base of the St. Cyrus andesites to Gourdon. In this 
accumulation of coarse, well water-worn material, with abundant intercalations 
of finer sandstone and occasional sheets of lava, there is the record of pro- 
longed and powerful denudation with intermittent volcanic activity. Dykes 
of a quartziferous porphyry cut the conglomerates, and at Gourdon they are 
pierced by the intrusion of serpentine above referred to. 
The proportion of andesite fragments in the conglomerates of this part 
of the coast varies, but is generally much lower than that of the rocks from 
the Highlands. Ihus at Johnshaven, out of 100 blocks, broken promiscu- 
ously from the conglomerate, I found that only 8 per cent were of andesite, 
while 44 per cent were of quartzite, and the remainder consisted of various 
quartz-porphyries, granites and schists. It is evident, therefore, that some 
area of crj’stalline rocks was subjected to enormous waste, and that its detritus 
was strewn over the floor of Lake Caledonia, at the same time that from the 
IMoutrose volcanic vents many streams of andesitic lava were poured forth. 
A vast mass of coarse conglomerate intervenes between Gourdon and 
Dunnottar, and forms a nearly continuous line of precipices which in some 
places rise 200 feet above the waves. The bedding is everywhere distinctly 
marked, so that there is no difficulty in following the succession of the strata, 
and estimating their thickness. From the last of the lavas at Gourdon 
to the base of the conglomerates near Stonehaven, there lies an accumulation 
of conglomerate at least 8000 feet thick. The boulders and pebbles in 
these deposits are generally well-rounded, and vary up to four feet or more 
in length. I oliserved one of quartz-porphyry at Kinneff which measured 
seven feet long and six feet broad. The proportion of andesite fragments 
in these conglomerates continues to be small. I ascertained that in the 
coarsest mass at Kinneff they numbered only 14 per cent ; at Todhead Point, 
a mile and a half to the north, 20 per cent, and at Caterline, three quarters 
of a mile further in the same direction, 21 per cent. 
In the midst of this gigantic accumulation of the very coarsest water- 
worn detritus, there are still records of contemporaneous volcanic action. 
Near Kinneff the beautiful andesite, with large tabular crystals of plagio- 
clase, alluded to on p. 274, occurs in the conglomerate.^ South of Caterline 
' For an analysis of the felspar in this rock, see Prof. Heddle’s paper. Tram. Soy. Soc. Edin. 
xxviii. (1879), p. 257. 
