The Old Red Sandstone

Abstract

AT a meeting of the Glasgow Geological Society on the 24th ult., Sir William Thomson, F.R.S., president, in the chair, Prof. Geikie, F.R.S., communicated the results of recent researches into the “History of the Deposits known as the Old Red Sandstone.” After a preliminary outlina of our present knowledge on the subject, he proceeded to consider the development of the Old Red Sandstone in the British Isles under its accepted three-bid division into Lower, Middle, and Upper. The Lower member, wherever its true base can be seen, is found to pass down conformably into the Upper Silurian rocks. But a well-marked line of demarcation, both by physical characters and fossil evidence, runs between the two systems. The Silurian formations continue replete with organic remains up to their uppermost zone; but on entering the red strata of the overlying system we meet with a remarkably abrupt change, for the fossils almost wholly disappear, and those owhich occur belong for the 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 15,000 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 runs from the North Sea at Stonehaven to the estuary of 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 conglomerates of Forfar, and an upper series of red and purple sandstones. They consist of thick sheets of various porphyries with beds of tuff and enormous masses of coarse volcanic conglomerates. Zones of grey flagstones, including 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.