472 



NATURE 



[April II, 1878 



Area of the Basins. 



1. The Old Red Sandstone tracts of the 



north of Scotland, embracing the 

 region of the Moray Firth, Caith- 

 ness, the Orkney Islands, the main- ) 

 land of Shetland, and perhaps part 

 of the south-westei'n coast of Nor- 

 way. J 



2. The central valley of Scotland be- 



tween the Highlands on the north 

 and the Silurian uplands on the 

 south, including the basin of the 

 Firth of Clyde, and ranging across 

 the north of Ireland to the high 

 grounds of Donegal. 



3. A portion of the south-east of Scot- 



land and north of England extend- 

 ing from near St. Abb's Head to 

 the head of Liddesdale, and includ- 

 ing the area of the Cheviot Hills, 



A. A district in the north of Argyllshire 1 

 extending fi-om the mouth of the i 

 Sound of Mull to Loch Awe, and ? 

 perhaps up into the southern part 

 of the Great Glen. '' 



5. The Old Red Sandstone region of | 

 Wales and the border counties of 

 England, bounded on the north and j^ 

 west by the older paleozoic hills, 

 the eastern and southern limits being 

 unknown. 



Short reference names 



proposed to be applied 



to them. 



Lake Orcadie. 



Lake Caledonia. 



Lake Cheviot, 



Lake Lome, 



The Welsh Lake, 



Lake Orcadie. — After describing the limits of this 

 basin, and giving a sketch of the labours of previous 

 observers in the Old Red Sandstone tracts of the north 

 of Scotland, the author proceeds to examine the evidence 

 for the threefold arrangement of the Old Red Sandstone 

 proposed by Murchison. He shows that nowhere are 

 the three groups. Lower, Middle, and Upper, found in 

 consecutive order ; that this so-called " Middle " division 

 occurs only in the north of Scotland, where it lies uncon- 

 formablyupon the older palaeozoic rocks, and is itself uncon- 

 formably overlaid by the Upper Old Red Sandstone, thus 

 occupying a position exactly similar to that of the Lower 

 Old Red Sandstone on the southern side of the High- 

 lands. He further points out that while some species of 

 fishes are common to the Old Red Sandstone on the two 

 sides of the Highland barrier, the lithological differences 

 between the deposits of the two areas are so great as to 

 make it evident that the rocks were laid down in distinct 

 basins and consequently that the fauna of each basin 

 might be expected to be more or less peculiar, as in many 

 analogous cases at the present day. As evidence that 

 adjacent areas in the time of the Lower Old Red Sand- 

 stone were strongly marked off from each other in their 

 faunas, reference is made to the contrast between the 

 fishes and crustaceans of the Welsh region and those of 

 Lanarkshire and Forfarshire, not a single species being 

 common to the two countries though some of the 

 genera are. Reasons are then given why the argument 

 used by Murchison from the occurrence of many of the 

 Scottish ichthyolites in Russia could not be regarded as 

 establishing the existence of a " Middle" division of the 

 Old Red Sandstone. 



The conclusion arrived at by the author is that the 

 Caithness flags or "Middle Old Red Sandstone" are 

 probably the general equivalents of the Lower Old Red 

 Sandstone of other regions, and that this system consists 

 in Britain of two well-marked divisions only — a Lower, 

 which graduates in some places into the Upper Silurian 

 rocks and is separated by an unconformability from an 

 Upper which in many districts passes up into the base of 

 the Carboniferous system. 



The various districts into which the area embraced 

 under the term Lake Orcadie may be divided are then 

 described seriatim. The detailed structure of Caithness 

 has been worked out by the author (partly with the co- 

 operation of his colleagues in the Geological Survey, Mr. 

 B. N. Peach and Mr. John Home) as affording the most 

 complete sections of the Old Red Sandstone in the North 

 of Scotland. Arranged in descending order, the various 

 stratigraphical zones stand as in the subjoined table : — 



Thickness 

 in feet. 



2000 



1000 



400 



5000 



5000 



9. John O'Groats Red Sandstone, Flagstones, and 

 impure Limestones and Shales 



8. Huna Flagstones, Shales, and Limestones ... 



7. Gill's Bay Red Sandstones 



6. Thurso or northern group of Flagstones, Shale-, 

 and Limestones 



5. Wick or eastern group of Flagstones, Shales, 

 and Limestones passing down into Red Shales 

 and Sandstones 



4. Dull Red Sandstones, Red Shales, and fine 

 Conglomerates 



3. Brecciated Conglomerates 



2. Badbea Red Sandstones and Shales or Clays.. 



I. Coarse basement Conglomei"ates 



16,200 ft. 



From the four lowest sub-divisions no fossils have yet 

 been obtained. The flagstones have yielded to Mr. C. W. 

 Peach, and other observers many land plants (some of 

 w^hich resemble forms described by Dawson from the 

 Gasp^ sandstones) as well as Estheria membranacea, 

 Pte7ygotus, sp., and many ichthyolites. Availing himself 

 of the list of localities furnished to him by Mr. Peach (to 

 whom he cordially acknowledges his obligations) with the 

 species of fish found at each, the author has constructed 

 a table of [the verdcal distribution of the fossil fishes in 

 Caithness. Some of the species range through almost 

 the entire succession of beds. Some, however, are either 

 peculiar to or very characteristic of one sub-division. 

 Thus Osteolepis arenatus and Dipterus Valenciennesi, are 

 not noted except from the group No. 5. In the Thurso 

 and the higher flagstones (Nos. 5, 8, and 9) Acanthodes, 

 Parexus, Cfieii'acatithus, Diplacanthus, Pterichthys, Tris- 

 tichopterus, and Holoptychius — genera absent from the 

 Wick beds — are found in greater or less abundance. 

 These strata are further marked by peculiar species of 

 genera which likewise occur among the older flagstones, 

 as Coccosteus pusillus and Osteolepis niicrolepidotus. 



The Orkney Islands are assigned to the higher sub- 

 divisions of the flagstone series, the protruding ridge of 

 granite and gneiss which rises at Stromness and Gremsa 

 being merely an indication of the irregular surface, on 

 which the deposits of Lake Orcadie were accumulated, 

 and of the slow progressive subsidence of the area. The 

 fossils, for which these islands have long been famous, 

 include most of those of the upper groups of Caithness, 

 with the addition of others which have been regarded as 

 distinct. In the determination of these fossils much skill 

 is required to discriminate between the accidental differ- 

 ences of aspect resulting from the condition of fossilisation. 

 The Orkney fishes, for instance, are preserved as black 

 jet-like impressions which, often very perfect when first 

 removed from the quarry, are apt to scale off, leaving in 

 each case only an amorphous layer which, though it 

 retains the contour of the fish, shows little or no trace of 

 structure. On the shores of the Moray Firth, on the 

 other hand, the organisms have been inclosed within 

 calcareous nodules ; their colours are sometimes briUiant, 

 and their scales, plates, fins, and bones, are often admi- 

 rably preserved and remain unchanged in the Museum. 

 Want of experience in these different modes of preserva- 

 tion may have led to a reduplication of species, especially 

 in the case of the Orkney and Moray Firth fishes. 

 Among the most interesting Orkney fossils is a portion 



