ART. XX.-Evolution of Geologic Climates; by CHARLES SCHUCHERT. Under the above title, F. H. Knowlton has written a most interesting paper,' the thesis of which is the following: "Relative uniformity, mildness, and comparative equability of climate, accompanied by high humidity, have prevailed over the greater part of the earth, extending to, or into, polar circles, during the greater part of geologic time-since, at least, the Middle Paleozoic. This is the regular, the ordinary, the normal condition" (p. 501). In another place we read: "By many it is thought that one of the strongest arguments against a gradually cooling globe and a humid, non-zonally disposed climate in the ages before the Pleistocene is the discovery of evidences of glacial action practically throughout the entire geologic column. Hardly less than a dozen of these are now known, ranging in age from Huronian to Eocene. It seems to be a very general assumption by those who hold this view that these evidences of glacial activities are to be classed as ice ages, largely comparable in effect and extent to the Pleistocene refrigeration, but as a matter of fact only three are apparently of a magnitude to warrant such designation. These are the Huronian glaciation, that of the 'Permo-Carboniferous,' and that of the Pleistocene. The others, so far as available data go, appear to be explainable as more or less local manifestations that had no widespread effect on, for instance, ocean temperatures, distribution of life, et cetera. They might well have been of the type of ordinary mountain glaciers, due entirely to local elevation and precipitation (pp. 547-548). And again: "If the sun had been the principal source of heat in pre-Pleistocene time, terrestrial temperatures would of necessity have been disposed in zones, whereas the whole trend of this paper has been the presentation of proof that these temperatures were distinctly non-zonal. Therefore it seems to follow that the sun-at least the present small-angle sun-could not have been the sole or even the principal source of heat that warmed the early oceans (p. 547). Doctor Knowlton naturally lays greatest stress in his conclusions upon the paleobotanic evidence, because it is this knowledge that he and his associates at Washington 'Bull. Geol. Soc. America, 30, 499-565, 1919. have so well in hand. However, that not all of his colleagues are in harmony with his views is shown by the following quotation from Wieland :2 "Does not a larger part of the Jurassic Ginkgo record also indicate wide climatic variation, second only in extent to that of the time of the Glossopteris flora? Would it not be singular if plant evidence remained wholly at variance from that of the insects and invertebrates, indicating climatic cooling in the late Trias and early Jura, not local in character?" And A. G. Nathorst is quoted by Wieland as saying that "during the time when the Ginkgophytes and Cycadophytes dominated, many of them must have adapted themselves for living in cold climates also. Of this I have not the least doubt." Knowlton knows that certain invertebrate evidence which he quotes from the writer and to which Wieland in the above quotation refers, indicates that toward the end of Triassic time and early in the Jurassic there were winters, about as they are now, in the latitudes of England to South Germany. In this connection it may be well to quote from still another paleontologist. Ulrich says:3 "Doubtless even in the Paleozoic there were times of relative frigidity when some of the higher parts of the marginal lands were ice-covered, in some instances attaining locally to glacial conditions. Here and there regular tillites are indicated, notably, as recently brought out by Dr. Edwin Kirk, in the Silurian deposits along the coast of Alaska." A careful reading of Knowlton's paper leaves the writer with the impression that the former holds that, except for the early Huronian, early Permian, and worldwide Pleistocene glacial times, the earth has been without temperature zones and large arid tracts, and that in general humidity has prevailed. Any paleontologist who is familiar with the climatic aspects of fossils will probably have to agree with Knowlton that the biotic evidence, and chiefly that of the floras, does in general bear out his conclusion that "climatic zoning such as we have had since the beginning of the Pleistocene did not obtain in the geologic ages prior to the Pleistocene." But what Knowlton actually holds is that there was "a non-zonal arrangement [of climate] prior to the Pleistocene" (p. 541), and that the temperature of the oceans was everywhere the same and without "wide-spread effect on the distribution of life" (p. 548). 2 G. R. Wieland, Amer. Jour. Botany, 7, 154, 1920. What Knowlton sees so well are the wide-spread fossil floras that usually occur in the middle portion of the geologic periods when the transgressions of the warm-water oceans over the continents are greatest; in other words, those times when the lands are smallest and are dominated by insular and therefore equable climates due to the wide-spread and warm oceanic waters. But what were the climatic conditions during the early and late parts of the many periods when the continents were largest, highest, and most arid? And it is at some of these times when the local or the widely spread tillites were forming! That the oceanic waters were not everywhere and during most of geologic time equably warm is attested by the varied life distribution seen in the large colonial foraminifers, stony corals, shelled cephalopods, and thick-shelled bivalves (chiefly the cemented forms) and gastropods. We see some or all of these groups of animals common in the far north and even in arctic waters during the Silurian, Devonian, Pennsylvanian, and Jurassic, but at other times they are either greatly reduced in variety in these regions or almost or completely absent. Take the Cretaceous foraminifers, sponges, corals, and rudistids, and the late (but not latest) Eocene nummulids, and see how they drop out of the record as one proceeds toward the Arctic. In India, nummulites attain to a diameter of eight inches, but in Japan they are all small, and north of Japan there are none of them at all. Moreover, the giants of any stock are rarely seen in far northern waters. These things can only mean temperature differences, and that the northern waters were frequently under 65° F. And if the oceanic waters were thus variable in temperature from time to time and from place to place, we may conclude that the climates of the lands were also varied and had zonal belts. On the other hand, when the lands are largest and the marine faunas localized in small sea-ways not widely accessible to the paleontologist, where are the cosmopolitan faunas and the large forms, and where is the variety and abundance of foraminifers, corals, bryozoa, cemented and thick-shelled molluscs, and shelled cephalopods? Exceptions there are, of course, but in general the restricted formations have small faunas and small species that are more or less unrelated to one another. In other words, we have here the stress-assemblages in which a prolifera tion of species and genera is beginning, along with the introduction of biotic lines that are to lead to the terminal giants. It is true that in these stress-faunas some of the handicaps of the environment are due to physical causes other than temperature, and yet the chief deterrent seemingly was the lack of the proper warmth. What kills off the giants? In some cases it is old age within the stock, but in many others it apparently is change in the environment, the deciding factor in which appears to be temperature. The evaluation of sediments for climatic purposes has not yet gone very far, nevertheless it is well known that limestones are far more readily deposited by the life of the warm waters than by those of cold ones. The great limestone-making areas of to-day are in the warm waters. Moreover it ought to be well known that the greatest amount of limestones, and especially of magnesian limestones and dolomites, does not occur in polar lands but rather in the temperate and subtropical ones. At times they are of extremely wide geographic distribution, as are the magnesian limestones and dolomites of the Ozarkian and early Ordovician, and those of the middle Silurian, middle Devonian, and middle and late Pennsylvaniantimes when the earth's climates are recognized by general agreement as mild throughout most of the northern hemisphere. Why is it that pure limestones and dolomites are less prevalent at other times during the Paleozoic and even within the latitude of the United States? In some cases it is undoubtedly because of the too great prevalence of muds and highlands, but in others it appears to be due to a temperature condition. In 1918, Blackwelder published a short but very striking paper entitled "The climatic history of Alaska from a new viewpoint." The viewpoint is that of the sediments, and one is impressed with the abundance he records of sombre and dark colors, muddy sandstones, and silts, and undecomposed mineral clastics, the scarcity of limestones, and the almost total absence of red colors. From the evidence recited Blackwelder concludes: "The combined evidence strongly suggests that the cool, moist climate of modern Alaska,-oscillating now and then toward the glacial Arctic condition on the one hand and toward the moist temperate on the other,-has been persistent, with but few real interruptions, throughout the known geologic history of Alaska." 'Eliot Blackwelder, Trans. Illinois Acad. Sci., 10, 275-280. When zoo-paleontologists have more clearly evaluated the climatic significance of the varied geographic marine faunas known to them, and geologists have learned the temperature import of the great variability in marine sediments, the conclusion will all the more certainly be drawn that the earth throughout the geologic ages has been subject to climatic variation. These variations will be seen to be slightest, indeed very slight, during the middle parts of the geologic periods when the world has almost no temperature belts; and variably greatest during their earliest and latest parts, when more or less marked climatic zones and even glacial climates were developed. To-day the variation on land between the tropics and the poles is roughly between 110° and -60° F., in the oceans between 85° and 31° F. In the geologic past the temperature for the greater parts of the periods of the oceans probably was most often between 85° and 55° F., while on land it may have varied between 90° and 0° F. At rare intervals the extremes were undoubtedly as great as they are to-day. The conclusion is therefore attained that throughout its history the earth has had temperature zones, varying from an intensity as marked as that of today to almost complete absence so that the greater part of the earth had an almost uniformly mild climate, without winters. |