OLD RED SANDSTONE OF WESTERN EUROPE. 303 
British Islands than exists elsewhere, and that the unravelling of its history 
will probably throw light upon most of the leading changes by which the west 
of Europe was affected during that interval of geological time. I shall in the 
present memoir restrict myself mainly to the Scottish tracts. 
For the sake of clearness in description, it will be of advantage to anticipate 
some of the conclusions which form the subject of discussion in the following 
pages. Thus, agreeing in the now very generally accepted view of the lacus- 
trine origin of the Old Red Sandstone, I shall speak of the separate basins of | 
deposit as /akes, to which, for ease of reference, different names will be given. 
As the history of the deposits in these basins often varies greatly, I shall adopt 
the geographical mode of treatment and describe each lake separately, with its 
varied accumulations, and the changes which they indicate. Viewed in a large 
way, the Old Red Sandstone of Great Britain naturally groups itself, strati- 
eraphically, into two divisions, which as a rule are strongly marked off from 
each other, both by physical structure and by difference of organic contents. 
These may be called Lower and Upper. I shall first trace the history of the 
various lakes of the earlier period. ‘To some extent these basins still remained 
during the later period, but the geography of the whole region had greatly 
altered in the interval. A sketch of these subsequent changes will show how 
the way was prepared for the Carboniferous system. 
THE LOWER OLD RED SANDSTONE. 
The Basins of Deposit in the British Area. 
Among the earlier Paleozoic formations, there is often such a marked per- 
sistence of lithological characters that the same group of rocks can be recognised 
with confidence in widely separated areas, even where the fossil evidence is 
scanty. A thin zone of black shale, found among the Llandeilo rocks of the 
south of Scotland, has been traced for more than a hundred miles, retaining its 
distinctive features all the way. The Wenlock and Ludlow shales, and mud- 
stones of the typical Silurian country, reappear with their familiar aspect in 
Westmoreland, Liddesdale, Kirkcudbright, Ayrshire, and Midlothian. "When 
we leave the marine formations, however, and pass from the top of the Upper 
Silurian groups into the Old Red Sandstone, no such general uniformity of 
stratification presents itself. On the contrary, with the accumulation of the 
deposits in limited basins, come local and often peculiar features, whereby even 
contiguous tracts are distinguished from each other. It is still possible roughly 
to make out with more or less clearness the limits of these basins, which seem 
sometimes to have been connected by narrow or shallow, and doubtless occa- 
sionally closed, water-channels; in other cases to have been completely isolated. 
VOL. XXVIII. PART II. ; 4Z 
