390 



NATURE 



\March i6, 1876 



most part to fishes and crustaceans. The author pointed 

 out the geological evidence in favour of great terrestrial 

 oscillations as explained by Prof. Ramsay, whereby the 

 bed of the Silurian sea in our area came to be raised into 

 land with wide lakes and inland seas. He showed that 

 the beginnings of the movements which led to those 

 results could be traced back a considerable way into the 

 Silurian period, that over large tracts the Silurian deposits 

 had been upraised before the close of that period, and 

 that the oscillations continued during the accumulation 

 of the Lower Old Red Sandstone, as indicated by the 

 coarse conglomerates, the great mass of the deposits, and 

 the unconformabilities traceable in them. Recent detailed 

 work of the Geological Survey has brought to light the 

 fact that this lower division of the Old Red Sandstone 

 attains an almost incredible thickness. In Lanarkshire and 

 Ayrshire Mr. B. N. Peach has found it to measure 1 5,000 feet. 

 In Perthshire, from the combined surveys of Mr. Peach and 

 Mr. R. L. Jack, it has been ascertained to reach a depth 

 of more than 19,000 feet. But the author has found that 

 traced eastward into Forfarshire and Kincardineshire, its 

 thickness rises above 20,000 feet. And yet in no case is 

 its top actually seen, since it has either been removed by 

 denudation, or buried under some more recent unconform- 

 able formation. Nor is its base to be found, since along the 

 flank of the Grampians a great fault nms from the North 

 Sea at Stonehaven to the estuaryof the Clyde, with the effect 

 of throwing the strata of the Old Red Sandstone on end, 

 sometimes for a distance of two miles from the line of the 

 dislocation. The amount of displacement must be in 

 some places not less than 5,000 feet, as indicated by the 

 position of occasional outliers of conglomerate on the 

 Highland side of the fault. One of the most striking 

 features in the formation is the enormous development of 

 its contemporaneous volcanic rocks. These are underlaid 

 in Kincardineshire by about 5,000 feet of sandstones and 

 shales, and they pass under the grey flags and conglome- 

 rates of Forfar, and an upper series of red and purple 

 sandstones. They consist of thick sheets of various por- 

 phyries with beds of tuff and enormous masses of coarse 

 volcanic conglomerates. Zones of grey flagstones, includ- 

 ing the well-known beds of Carmylie near Arbroath, are 

 intercalated in them. In the Ochil Hills, according to 

 the measurements of Mr. B. N. Peach, this volcanic 

 zone reaches a depth of not less than 6,500 feet. It runs 

 from the sea-coast at Dunottar through the chain of the 

 Sidlaw and Ochil Hills to near Stirling. It reappears 

 south of the Forth, in the Pentland chain, and stretches 

 south-westwards in great force across Lanarkshire and 

 Ayrshire. The author then alluded to the fossils hitherto 

 noticed in this part of the Old Red Sandstone in different 

 parts of Britain, pointing out the contrast they present to 

 those of the preceding Silurian rocks. He showed that 

 in Forfarshire the well-known crustaceans and fishes had 

 been obtained from strata, lying not as hitherto supposed 

 at the base of the system, but several thousand feet 

 higher, and that the fish-bed found by Mr. Mitchell in 

 Kincardineshire, and supposed by Sir Roderick Murchison 

 to indicate from its Acanthodian forms an approach to 

 the middle Old Red Sandstone, really lay below the 

 position of the Turin flagstones so well explored by Mr. 

 Powrie. 



The so-called "Middle" Old Red Sandstone is not 

 known certainly to exist anywhere else in Britain than in 

 the north of Scotland. This subdivision was introduced 

 by Sir R. Murchison, and is based wholly on the evidence 

 from fossils. It presents a remarkably distinct series of 

 ichthyolites, which have not been met with in the Lower 

 Old Red Sandstone of the south, and which have therefore 

 been held to mark a higher series of deposits. The 

 " Middle " Old Red Sandstone is typically developed in 

 the well-known flags of Caithness. Those strata, long 

 since described by Sedgwick and Murchison, cover nearly 

 the whole of that county, and stretch into the Orkney 



and Shetland Islands. The author had measured a 

 section of these on the east coast of Caithness more than 

 8,000 feet in thickness. They show conglomerates and 

 red sandstones at the base, and similar strata re- 

 appear on different higher horizons. But on the 

 whole, the series consists of dark-grey, hard, fine- 

 grained flagstones, sometimes bituminous, often cal- 

 careous, and frequently abounding in remains of fishes, 

 entomostraca, and land-plants. No evidence of contem- 

 poraneous volcanic action has yet been met with in these 

 rocks. The general character of the whole series differs 

 in many respects from that of the Old Red Sandstone on 

 the south side of the Grampians, and appears to indicate 

 widely different conditions of deposit. The basins in 

 which the Caithness flags, and the Arbroath flags accu- 

 mulated, were separated by the intervening mass of the 

 Inverness-shire and Aberdeenshire highlands, as shown by 

 the basement conglomerates on each side of the moun- 

 tains. The author then dwelt on the fossil evidence and 

 its bearings. He suggested that it could not be held to 

 prove a "middle" series, and that it was not inconsistent 

 with the idea that the Caithness flags really belonged to 

 the Lower Old Red Sandstone, the peculiarities of their 

 fauna not being greater than might be due to great 

 differences of physical geography, and to the fact that 

 the respective areas of deposit were isolated from each 

 other. He further pointed out that some of the most 

 characteristic forms of the lower group occur in the 

 Caithness and Orkney beds, e.g., Pteraspts, and Ptery- 

 gptus. The Caithness flags abound in land-plants like 

 Sigillaria, Lxpidode7idron, and some of the peculiar De- 

 vonian forms found by Dr. Dawson in Gaspd. Some of 

 these latter forms have recently been detected by Mr. R. 

 L. Jack in the course of the work of the Geological Survey 

 in Perthshire. 



The red and yellow sandstones, red clays, and red con- 

 glomerates and breccias, included under the term " Upper 

 Old Red Sandstone," are copiously developed in Wales, 

 in Ireland, and in the south of Scotland. In the two 

 last-named regions they have been shown to lie uncon- 

 formably on all older formations, there being a complete 

 physical discordance, and an entire difference in organic 

 contents, between these strata and the Lower Old Red 

 Sandstone. In South Wales a less marked break in the 

 series may be suspected from the cautious descriptions 

 given by Sir Henry De la Beche. In the north of Scot- 

 land Sir R. Murchison has described the Upper as gradu- 

 ating downwards into the Middle or Caithness flags. 

 In Caithness itself, however, and in Orkney, they are 

 completely discordant, and the same relation may be 

 inferred to hold elsewhere. This uppermost member 

 thus bears the same relation to the Caithness flags as to 

 the Arbroath flags. Wherever the top of the series can 

 be seen, it is found to pass gradually and conformably 

 into the base of the Carboniferous system. So thoroughly 

 do these two series of deposits dovetail with each other that 

 no sharp line can be drawn between them. If we work 

 our way into the red rocks from the Carboniferous side, 

 we may claim them as merely the base of the Carbonife- 

 rous system. If we approach them from the side of the 

 Old Red Sandstone, we may well regard them as a late 

 and unconformable sub-division of this system. 



The author next adverted to the fossils peculiar to the 

 Upper Old Red Sandstone, calling attention to the con- 

 tinuance of land-plants and ganoid fishes as characteristic 

 Old Red Sandstone features. He then, in conclusion, 

 pointed out the physical geography which appears to be 

 indicated by the deposits of this period. It is still pos- 

 sible to map out some of the terrestrial and lacustrine 

 areas which then marked the site of Britain. Hill-ranges, 

 still in existence, formed prominent features in the land- 

 scapes of that time, though with many differences of out- 

 line ; in particular, with water-filled bays, straits, long 

 fjord-hke inlets and scattered lakes which have been filled 



