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638 MR ARCHIBALD GEIKIE ON THE CHRONOLOGY 
sometimes by masses of syenite or granite. While conducting the Geological 
Survey of the Lammermuir Hills during the summer of 1859, the number of these 
igneous protrusions astonished me. In nearly every glen I found the Silurian 
strata traversed by dykes, sometimes very small, and never very large, except in 
one or two instances. In some localities, indeed, the vertical grits and hardened 
shales seemed riddled, as it were, with interjected veins, and these, as a general 
rule, were observed to occur sporadically,—so much so, that when, after passing 
over a space free from dykes, | at last came upon one or two together, it could 
generally be predicted that a congregated group of veins and irregular knobs 
would be found in close proximity. 
All these igneous rocks are intrusive ; and in noting their varieties of colour 
and composition, their vast numbers, and their multiform intersections through 
the Silurian strata, I could not but wish to obtain, if possible, some clue to their 
date. For this purpose I examined the conglomerates of the Upper Old Red series, 
by which the Silurian strata of this region are unconformably covered. It re- 
quired no long or careful search to discover among the rounded and subangular 
fragments of these conglomerates abundance of felspathic pebbles, often of con- 
siderable size, and plainly referable to the numerous dykes of the neighbourhood. 
Hence it became evident that the igneous rocks in question were older than the 
Upper Old Red Sandstone. And this conclusion received further confirmation 
from the fact that, though the Silurians are everywhere intersected by felspathic 
dykes, such dykes do not penetrate into the overlying Old Red Sandstone. After 
a survey of the whole of Lammermuir, I have discovered only four trifling 
felstone dykes among the Old Red Sandstone strata, while, in the Silurian grits 
and shales immediately adjoining, they may be counted by the score and the 
hundred. There could be no doubt, therefore, that the felspathic porphyries of 
Lammermuir date anterior to the deposition of the Upper Old Red Sandstone, and 
posterior to that of the Lower Silurian. The interval between these two forma- 
tions was very great. It seemed possible, however, to define the age of the dykes 
more closely. 
With this aim I sought, as a preliminary step, to connect the Old Red Sand- 
stone of Lammermuir with the same formation in other parts of South Scotland, 
and to determine how far the Upper Old Red Sandstone descended in regular 
sequence towards the Silurian system. The district selected as best calculated to 
throw light upon the matter was the neighbourhood of Lesmahagow, along the 
borders of Lanark and Ayrshire. 
The results of this examination I have described in a paper read before the 
Geological Society, 18th January 1860.* They may be briefly summed up in the 
statement that the Upper Silurian and Lower Old Red Sandstone, which form one 
* See Quart, Jour. Geol. Soc., vol. xvi. p. 312, 

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