348 PROFESSOR GEIKIE ON THE 
sion was complete. MuRcHISON ingeniously contended that the great hiatus 
shown by me to exist between Lower and Upper Old Red Sandstone could 
be bridged over by his Middle or Caithness Flag group of the north; and he 
cited the Devonian rocks of Russia, where a number of the fishes of the Old 
Red Sandstone of the north of Scotland are associated with marine shells belong- 
ing to Middle Devonian types. The introduction of a middle Old Red Sandstone 
into the series was described as a “ masterly suggestion,” and as “the greatest 
advance made of late years in the classification of the British Devonian rocks.”* 
I shall venture in a later part of this essay to show grounds for doubting 
whether such a middle Old Red Sandstone really has any existence. The three 
groups not only do not occur in any one continuous section, they are not even 
met with together in the same region. I shall show that even as far north as 
Orkney the same twofold grouping with intervening unconformability can be 
seen, which is so persistent in other parts of the island. 
But, apart from questions of classification, there are features of such peculiar 
interest connected with the Old Red Sandstone as to give that series of rocks a 
claim for much more thorough investigation than it has yet received. From 
these venerable deposits we obtain some of the earliest traces of land on the sur- 
face of the globe. They bring before us, dimly it is true but still certainly, por- 
tions of the Paleozoic continent which preceded our modern Europe. We can 
make out from them the positions of a few of the great lakes of that time, and can 
trace the sites of some of the larger rivers. We have fragments of the vegeta- 
tion which covered the land, and can in some measure realise the nature of the 
life which teemed in some of the sheets of water, or found only a precarious 
subsistence in others. 
No attempt has yet been made, by working out in detail the stratigraphical 
order of the various Old Red Sandstone tracts of the British Islands, to present 
a connected view of their relations to each other, and of the history which they 
record. Occurring in detached areas, the Old Red Sandstone of this country has 
been very commonly looked upon as a very fragmentary formation. Its enor- 
mous depth, and the remarkable vicissitudes of physical geography which it 
has chronicled, are still but vaguely appreciated, even by British geologists. 
There need be no wonder, therefore, that its importance has not been recognised 
by our fellow-workers in other countries, or that we should find an able writer 
on the other side of the Atlantic cautioning his readers “that they should not 
measure the Erian (Devonian) formations of America or the fossils which they 
contain by the comparatively depauperated representation of this portion of the 
geological scale in Europe.”t I venture to affirm that a better comparison of 
* Salter, “Quart. Journ. Geol. Soc.” vol. xix. p. 493. 
+ J. W. Dawson on “ Fossil Plants of the Devonian and Upper Silurian Formations of Canada.” 
—“ Geol. Survey of Canada Memoirs,” 1871, p. 10. 

