A century of science in America

by Unknown

In South Africa, beneath the marine Lower Devonian, occurs the Table Mountain series, 5000 feet thick. The series is essentially one of quartzites, with zones of shales or slates and with striated pebbles up to 15 inches long. The latter occur in pockets and seem to be of glacial origin. There are here no typical tillites, and no striated undergrounds have so far been found. While the evidence of the deposits appears to favor the conclusion that the Table Mountain strata were laid down in cold waters with floating ice derived from glaciers, it is as yet impossible to assign these sediments a definite geologic age. They are certainly not younger than the Lower Devonian, but it has not yet been established to what period of the early Paleozoic they belong.

In southeastern Australia occur tillites of wide distribution that lie conformably beneath, but sharply separated from the fossiliferous marine Lower Cambrian strata. David (1907), Howchin (1908), and other Australian geologists think they are of Cambrian time, but to the writer they seem more probably late Proterozoic in age. In arctic Norway Reusch discovered unmistakable tillites in 1891, and this occurrence was confirmed by Strahan in 1897. It is not yet certainly known what their age is, but it appears to be late Proterozoic rather than early Paleozoic. Other undated Proterozoic tillites occur in China (Willis and Blackwelder 1907), Africa (Schwarz 1906), India (Vredenburg 1907), Canada (Coleman 1908), and possibly in Scotland.

The oldest known tillites are described by Coleman in 1907, and occur at the base of the Lower Huronian or in early Proterozoic time. They extend across northern Ontario for 1000 miles, and from the north shore of Lake Huron northward for 750 miles.

Fossils as Climatic Indexes.—Paleontologists have long been aware that variations in the climates of the past are indicated by the fossils, and Neumayr in 1883 brought the evidence together in his study of climatic zones mentioned elsewhere. Plants, and corals, cephalopods, and foraminifers among marine animals, have long been recognized as particularly good “life thermometers.” In fact, all fossils are climatic indicators to some extent, and a good deal of evidence concerning paleometeorology has been discerned in them. This evidence is briefly stated in the paper by Schuchert already alluded to, and in W. D. Matthew’s Climate and Evolution, 1915.

Sediments as Climatic Indexes.—Johannes Walther in the third part of his Einleitung—Lithogenesis der Gegenwart, 1894—is the first one to decidedly direct attention to the fact that the sediments also have within themselves a climatic record. In America Joseph Barrell has since 1907 written much on the same subject. On the other hand, the periodic floodings of the continents by the oceans, and the making of mountains, due to the periodic shrinkage of the earth, as expressed in T. C. Chamberlin’s principle of diastrophism and in his publications since 1897, are other criteria for estimating the climates of the past.

Conclusions.—In summation of this subject Schuchert says:

“The marine ‘life thermometer’ indicates vast stretches of time of mild to warm and equable temperatures, with but slight zonal differences between the equator and the poles. The great bulk of marine fossils are those of the shallow seas, and the evolutionary changes recorded in these ‘medals of creation’ are slight throughout vast lengths of time that are punctuated by short but decisive periods of cooled waters and great mortality, followed by quick evolution, and the rise of new stocks. The times of less warmth are the miotherm and those of greater heat the pliotherm periods of Ramsay.

On the land the story of the climatic changes is different, but in general the equability of the temperature simulates that of the oceanic areas. In other words, the lands also had long-enduring times of mild to warm climates. Into the problem of land climates, however, enter other factors that are absent in the oceanic regions, and these have great influence upon the climates of the continents. Most important of these is the periodic warm-water inundation of the continents by the oceans, causing insular climates that are milder and moister. With the vanishing of the floods somewhat cooler and certainly drier climates are produced. The effects of these periodic floods must not be underestimated, for the North American continent was variably submerged at least seventeen times, and over an area of from 154,000 to 4,000,000 square miles.

When to these factors is added the effect upon the climate caused by the periodic rising of mountain chains, it is at once apparent that the lands must have had constantly varying climates. In general the temperature fluctuations seem to have been slight, but geographically the climates varied between mild to warm pluvial, and mild to cool arid. The arid factor has been of the greatest import to the organic world of the lands. Further, when to all of these causes is added the fact that during emergent periods the formerly isolated lands were connected by land bridges, permitting intermigration of the land floras and faunas, with the introduction of their parasites and parasitic diseases, we learn that while the climatic environment is of fundamental importance it is not the only cause for the more rapid evolution of terrestrial life....

Briefly, then, we may conclude that the markedly varying climates of the past seem to be due primarily to periodic changes in the topographic form of the earth’s surface, plus variations in the amount of heat stored by the oceans. The causation for the warmer interglacial climates is the most difficult of all to explain, and it is here that factors other than those mentioned may enter.