WCP2165

[1]¹

Perth

30^th Nov. 1879

My dear Croll²

I return M^r Wallace's letter. I don't know that any one has sought about the older formations for palaeontological evidence of glacial and interglacial periods. Several geologists have indeed stated that they do not believe any such evidence exists. It is very doubtful, however, whether they could recognize it even if did exist. I am afraid that the physical evidence is what we are most likely to obtain — and that as you are aware is not so insignificant as some seem to think. For example, there are the great erratics which occur in the Lower Silurian of South Ayrshire. Those surely point to the presence somewhere of glaciers — perhaps in the north west of Scotland. Perhaps also the general absence of organic remains throughout great thickness, of the argillaceous Silurian strata of Scotland may point to unfavourable conditions — such as might be induced by the influx of vast bodies of cold fresh water and the presence of ice. Then we have the great conglomerates of the Old Red Sandstone formations. Their distribution throughout the country and their relation to the Old [2] Silurian land point unmistakeably I think to the presence in the old valleys of considerable glaciers. Ramsay³ long ago recognized the resemblance of the Old Red Conglomerates of South of Scotland to boulder-clay. My brother⁴ as you know got striated stones in the conglomerates of the Lammermuir Hills and my survey of the Cheviot Hills led me to the conclusion that the conglomerates which wrap round the foot-slopes of these hills were simply diluvial gravels, swept down by torrent from the snow-fields and glaciers into the wide lake which then washed covered a broad area in the Tweed country. It is perhaps not without significance that the lacustrine deposits ^{I refer to} are for the most part barren of organic remains. Similar conglomerates occur in the Lower Carboniferous of Scotland, and I have sometimes had a suspicion that the absence of organic remains throughout considerable thickness of shale and sandstone in our Carboniferous Limestone series and even in the true coalmeasure themselves may have a deeper meaning than palaeontologists have been aware of. Indications of floating ice (erratics) occur as you know in the Carboniferus strata of North America. The Permian breccias as Ramsay holds seems to be decidedly glacial. Even in the Secondary formations traces of ice action have been noted as for example the course breccias & conglomerates in the oolite of Luther [3] land — the erratics in the chalk of Croydon [illeg.] then we have the great erratics in the Eocene of the Alps, and the coarse bouldery shingle of the Swiss molasse and the huge erratics in the Miocene of Northern Italy. You might refer M^r. Wallace to Note A in the Second Edition of my Great Ice Age⁵ where he will find reference to the papers bearing upon this subject. It is certainly much to be desired that palaeontologists would let themselves to a closer sounding of the evidence. They have now and again described the dwarfed conditions of the organisms met with in at certain horizons in the strata of particular formations — but then have been usually explained. I think, by the influx of muddy water etc., and I am not prepared to deny that this may be ^the true explanation. In one Scotch Carboniferous Limestone — the coral and molluscs are largest and best developed in these limestones, which are purest — in the highly argillaceous limestones they are often dwarfed. But we know so very little of the geographical zoology and botany of the older formations that it seems extremely doubtful whether palaeontologists have got sufficient data to enable them to detect the evidence of changing climatic conditions in the appearance or disappearance or relative abundance or paucity of any particular groups of fossils. The evidence may exist although we want the knowledge to see it. And the same is true to a large extent with Secondary and [4] even with Tertiary formations. Look what miserable fragments now exist of the Miocene lands. If it had not been for the fact that the Miocene was a period of great volcanic activity in the north and that old land surfaces became sealed up and preserved under enormous accumulations of lava etc — how much should we have known of the conditions of the Northern Regions in Miocene times? Notwithstanding all this I feel that M^r. Wallace is right in thinking that if a great succession of strata such as that of the Oolite of England were closely scrutinised palaeontologists might yet discover traces of changing climate conditions. The inquiry would be laborious and would necessarily have to be exhaustive — but I have great hopes that it would result in the producing additional evidence in favour of your theory. I have long been gathering the facts relative to our Scotch Carboniferous Formation but have never yet found time to work this matter up. But so soon as I get through with what I have in hand I mean to set about that work in earnest.

I am afraid that what I have written may be rather confused, but I scribble under the disadvantages of a bad cold and heavy headache.

Ever sincerely yours | James Geikie⁶ [signature]