One cannot exclude the effect of nebulae, whose galactic distribution is obviously nonrandom. It is well known that the cold dust matter forms tremendous concentrations whose dimensions sometimes reach many parsecs. Possibly, a special significance in this respect attaches to the so-called globules which contain the highest dust densities and consequently possess the lowest translucence. Even if one accepts the average theo retically possible magnitude of light absorption (instead of the maximum one) during the passage of the Sun through the zone of a nebula, the variation in the amount of solar energy reaching the Earth will be sufficient to account for a significant decline of the mean annual temperature on the Earth's surface.
Thus, the predominant concentration of fine dust matter in certain parts of the Galaxy, and the traversing by the solar system of a zone of concen tration of cosmic dust over fairly constant sections of the galactic orbit, could satisfactorily explain the periodic nature of the great coolings in the Earth's history and the related intensive development of ice sheets, i.e., the periodicity of cosmic winters.
In conclusion I should like to touch upon another highly specific subject. Without advancing any definite interpretation of the phenomenon, I permit myself to draw the investigators' attention to a circumstance that appears to be significant, namely, the possible asymmetry in the development of the great ice sheets.
The Upper Pliocene and the Quaternary glaciations mostly affected the northern hemisphere. On the other hand, an overwhelming majority of traces of the Gondwana glaciation are concentrated in the southern hemi sphere, with the exception of India which, however, also tends toward the southern continental blocks in its general paleogeographic situation (Wegener). The Ordovician glaciation has not yet been adequately studied, but there are grounds to assume that it mainly affected the northern hemisphere. There are at least two glaciation cycles within the extremely long geological period corresponding to the so-called Algonkian and Eocambrian. Obviously, the Adelaide glaciation (Australia) may serve as a typical representative of the youngest of these glaciations, while Keweenaw (North America) and possibly the Patom Plateau in Siberia (?) are typical of the oldest glaciation.
As yet, very little information is available on the relative stratigraphy of the Middle and especially the Lower Proterozoic and even less on the Archean. Yet, here too there seems to be a definite regularity in the
arrangement of glaciation traces beginning with the enormous Huronian glaciation of the Canadian Shield (Lower Proterozoic) and the possibly synchronous boulder puddings of the Barangulskaya suite of the Ural-Tau zone (South Urals) and ending with the supposedly Upper Pre-Cambrian tillites of Damaraland, Windhoek, Rehoboth, and certain adjacent territories in South Africa.
The natural sciences are standing on the brink of a new and important stage in their development. This refers not only to physics but also largely to the entire cycle of the geologico-geographical sciences. Some of the most daring and the most disputed ideas and hypotheses advanced by investigators of past generations are finding solid foundations in the data of the most recent investigations. In this respect it is sufficient to point to the fate of Wegener's ideas.
The evolution undergone by the natural sciences in their development is noteworthy and instructive. The age of encyclopedic knowledge and universal concepts was superseded, in the second half of the nineteenth and the beginning of the twentieth century, by a period of the utmost differ entiation and separation of the sciences. The new phase that can be currently discerned marks the convergence and cooperation of the most diverse scientific disciplines which, without mutual assistance, cannot provide satisfactory solutions of the most significant and difficult problems which they face.
Never before has such a multitude of unanswered questions arisen in geological science as in recent years, and never before was the possibility of solving these problems, in light of the brilliant successes of physics, astronomy, and especially engineering, so great.
There is no danger of overestimating the importance of the turning point which is occurring before our eyes. The geocentric view of the nature of phenomena that has stubbornly dominated the minds of a majority of natural philosophers (with only a few exceptions; the reader is reminded of Humboldt's "cosmos") has now been destroyed. The main effort has been made, and thought has been liberated from the hypnotic influence of centuries-old traditions. Possibly for the first time since Galilei the Earth has acquired a place of its own in the cosmos, not as a planet but as a complex geological body, and the geological processes have become an inalienable part of the general life of the universe.