OLD RED SANDSTONE OF WESTERN EUROPE. 367 
to our knowledge of the distribution of the ichthyolites in the Old Red Sand- 
stone north of the Grampians. To some of the more important of these I shall 
make reference in subsequent pages. But as regards the state of opinion on 
the classification of the Old Red Sandstone, it remains very much as MuRCHISON 
left it. The threefold grouping is received without question, and the Caithness 
flagstones are acknowledged to form a great middle series. 
My own labours in the north began with the endeavour to ascertain the 
relation between these so-called “middle” beds and the upper division with 
Holoptychius, &c. In working out this point I have been gradually led to 
extend my observations over most of the area of what I have termed Lake 
Orcadie. In particular I found it necessary to make out in some detail, with 
the co-operation of my friend and colleague in the Geological Survey, Mr 
B. N. Peacu, the order of succession among the flagstones of the typical 
Caithness area, and to ascertain, as far as the available evidence would permit, 
the extent to which paleontological subdivision could be introduced into them. 
This task had never been attempted, and yet, until it had been in some measure 
at least accomplished, it was obviously hopeless to undertake any correlation of 
the fragmentary portions of the Old Red Sandstone of the north of Scotland, 
and still less any comparison between them and the area south of the Gram- 
pians. Over a large part of Caithness I have been accompanied and assisted 
by Mr B. N. Praca, who likewise took part with me in traverses of Orkney 
and Shetland, and to whose skill and energy as an observer, always cordially 
given, Iam under many obligations. Another Survey colleague, Mr Joun Horne, 
accompanied and assisted me in a traverse of the coast of Caithness, between 
Thurso and Duncansbay Head, as well as in the southern and western parts of 
the county. To the unravelling of the history of Lake Caledonia, on which I 
had been engaged for many years previously, a singular fascination had 
attached. I soon felt that a similar impulse was given by the study of the 
northern tracts. It led me to the shores of the Moray Firth and of Caithness 
summer after summer, and through the Orkney Islands to the remote Shetland. 
In now presenting the result of these journeys I am well aware that it cannot 
be regarded as more than a first sketch of the subject, that some parts are 
much less fully worked out than others, and that many miodifications and cor- 
rections may eventually be needed throughout. I have myself, however, so 
long desired to possess such a first general outline for my own guidance that I 
venture to hope it will not be without interest and usefulness to geologists who 
are studying the history of the paleozoic rocks of this country. 
The area which I have included under the general name of Lake Orcadie 
may be conveniently divided into four geographical districts,—1st, Caithness 
and Sutherland; 2d, the Orkney Islands; 3d, the Shetland Islands; and 4th, the 
Basin of the Northern Firths. In the north of Caithness and throughout the 
