As yet, the vast factual data amassed by geologists has been only partially utilized in specialized paleoclimatic analyses. As an example De-Geer's monumental review (1940) with its numerous varve-correlation diagrams may be cited; studies of it may yield very rich material on Upper Glacial climatic variations in Sweden.
4. Apart from the rhythmic banding due to exogenic factors (primarily climatic variations) there also exists a rhythmic banding of endogenic origin which is most clearly manifested by the structure of carbonate or siliceous sediments formed by Recent and Quaternary thermal springs (Figure 7 and 8) as well as during older geological periods. This is vividly illustrated by the banding of jaspers among the volcanogenic Devonian deposits in the South Urals and Altai (Figure 9). Banding of endogenic type has not yet been adequately studied. However, the available data suggest that the rhythms of endogenic banding generally obey the same regularities as do the exogenic rhythms.
5. In many cases the climatic periods established by geologists agree remarkably well with periodicities of cosmic phenomena established by astronomers. These include the fairly well-studied periodic variations in the intensity of solar radiation. This probably explains not only the minor rhythms (which include, among others, the period of 11 to 12 years that has been known to many geologists and paleogeographers for a long time), but also climatic cycles of considerably longer duration, such as the major stages in the climatic variations during the glacial periods and corresponding variations in the development of sheet glaciers. Traces of such stages can be perceived in the solar radiation curves for the last 600,000 years constructed by Milankovitsch.
6. We are still in complete ignorance of the nature of the largest climatic variations on a scale of hundreds, thousands, and millions of years. One may only speak, with some measure of certainty, of the reflection of the complete galactic revolution of the solar system in the general course of geological processes and climatic evolution of the Earth. This period of revolution, which is 175 to 180 million years according to Parenago, is probably paralleled by the incidence of tremendous terrestrial glaciations which have been termed the "cosmic winters". This subject will be discussed in
somewhat greater detail below.
The existence of geological (and probably also climatic, in the broad meaning of this word) cycles of even greater duration, com prising several periods of complete galactic revolutions, i.e., as much as 700 to 1000 million years, may be considered. attention should be drawn to the problem of the "Upper Pre-Cambrian" which has recently been considered from a new, though far from unquestionable, slant. The latest absolute geochronological data based mainly on studies of the Pre-Cambrian on USSR territory indicate, fairly consistently, the tremendous duration of time consumed in the accumulation of the so called "Riphean", or "Sinian"* complex, of about 700 million or even a billion years.
The field geologists, thoroughly acquainted with these strata, have been amazed by their absolute dating, although it can hardly be reconciled with the accepted concepts of the typical physiographic and geological conditions of the Upper Pre-Cambrian. These include the relatively small average thickness of strata and the Eocambrian predominance of very rapidly accumulated continental and shallow marine sediments, such as strata of cross grained sandstones, reef limestones, con glomerates, tillites, etc.; the absence of regionally consistent lacunes and traces of large diastrophic phases in Eocambrian sections; and finally the insignificant evolution of the organic nature mainly represented by the blue green algae, whose general habit remained essentially unchanged throughout the entire Upper Pre-Cambrian, including not only its uppermost horizons but partly the Lower Cambrian as well.
In this connection it is important to note that minor rhythms of climatic variations, including the secular rhythm, are excellently reflected in the Upper Pre-Cambrian deposits, especially in varved shales which are a facies of periglacial lakes (South Urals, Yenisei Range, Patom Plateau, etc.).