1. The irregular duration of the interval from one glacial epoch to another corresponds with the irregular distribution of the stars. If glaciation is indirectly due to stellar influences, the epochs might fall close together, or might be far apart. If the average interval were ten million years, one interval might be thirty million or more and the next only one or two hundred thousand. According to Schuchert, the known periods of glacial or semi-glacial climate have been approximately as follows:
LIST OF GLACIAL PERIODS
1. Archeozoic.
(1/4 of geological time or perhaps much more)
No known glacial periods.
2. Proterozoic.
(1/4 of geological time)
a. Oldest known glacial period near base of Proterozoic in Canada. Evidence widely distributed.
b. Indian glacial period; time unknown.
c. African glacial period; time unknown.
d. Glaciation near end of Proterozoic in Australia, Norway, and China.
3. Paleozoic.
(1/4 of geological time)
a. Late Ordovician(?). Local in Arctic Norway.
b. Silurian. Local in Alaska.
c. Early Devonian. Local in South Africa.
d. Early Permian. World-wide and very severe.
4. Mesozoic and Cenozoic.
(1/4 of geological time)
a-b. None definitely determined during Mesozoic, although there appears to have been periods of cooling (a) in the late Tria.s.sic, and (b) in the late Cretacic, with at least local glaciation in early Eocene.
c. Severe glacial period during Pleistocene.
This table suggests an interesting inquiry. During the last few decades there has been great interest in ancient glaciation and geologists have carefully examined rocks of all ages for signs of glacial deposits. In spite of the large parts of the earth which are covered with deposits belonging to the Mesozoic and Cenozoic, which form the last quarter of geological time, the only signs of actual glaciation are those of the great Pleistocene period and a few local occurrences at the end of the Mesozoic or beginning of the Cenozoic. Late in the Tria.s.sic and early in the Jura.s.sic, the climate appears to have been rigorous, although no tillites have been found to demonstrate glaciation. In the preceding quarter, that is, the Paleozoic, the Permian glaciation was more severe than that of the Pleistocene, and the Devonian than that of the Eocene, while the Ordovician evidences of low temperature are stronger than those at the end of the Tria.s.sic. In view of the fact that rocks of Paleozoic age cover much smaller areas than do those of later age, the three Paleozoic glaciations seem to indicate a relative frequency of glaciation. Going back to the Proterozoic, it is astonishing to find that evidence of two highly developed glacial periods, and possibly four, has been discovered. Since the Indian and the African glaciations of Proterozoic times are as yet undated, we cannot be sure that they are not of the same date as the others. Nevertheless, even two is a surprising number, for not only are most Proterozoic rocks so metamorphosed that possible evidences of glacial origin are destroyed, but rocks of that age occupy far smaller areas than either those of Paleozoic or, still more, Mesozoic and Cenozoic age. Thus the record of the last three-quarters of geological time suggests that if rocks of all ages were as abundant and as easily studied as those of the later periods, the frequency of glacial periods would be found to increase as one goes backward toward the beginnings of the earth"s history. This is interesting, for Jeans holds that the chances that the stars would approach one another were probably greater in the past than at present.
This conclusion is based on the a.s.sumption that our universe is like the spiral nebulae in which the orbits of the various members are nearly circular during the younger stages. Jeans considers it certain that in such cases the orbits will gradually become larger and more elliptical because of the attraction of one body for another. Thus as time goes on the stars will be more widely distributed and the chances of approach will diminish. If this is correct, the agreement between astronomical theory and geological conclusions suggests that the two are at least not in opposition.
The first quarter of geological time as well as the last three must be considered in this connection. During the Archeozoic, no evidence of glaciation has yet been discovered. This suggests that the geological facts disprove the astronomical theory. But our knowledge of early geological times is extremely limited, so limited that lack of evidence of glaciation in the Archeozoic may have no significance. Archeozoic rocks have been studied minutely over a very small percentage of the earth"s land surface. Moreover, they are highly metamorphosed so that, even if glacial tills existed, it would be hard to recognize them.
Third, according to both the nebular and the planetesimal hypotheses, it seems possible that during the earliest stages of geological history the earth"s interior was somewhat warmer than now, and the surface may have been warmed more than at present by conduction, by lava flows, and by the fall of meteorites. If the earth during the Archeozoic period emitted enough heat to raise its surface temperature a few degrees, the heat would not prevent the development of low forms of life but might effectively prevent all glaciation. This does not mean that it would prevent changes of climate, but merely changes so extreme that their record would be preserved by means of ice. It will be most interesting to see whether future investigations in geology and astronomy indicate either a semi-uniform distribution of glacial periods throughout the past, or a more or less regular decrease in frequency from early times down to the present.
2. The Pleistocene glacial period was divided into at least four epochs, while in the Permian at least one inter-glacial epoch seems certain, and in some places the alternation between glacial and non-glacial beds suggests no less than nine. In the other glaciations the evidence is not yet clear. The question of periodicity is so important that it overthrows most glacial hypotheses. Indeed, had their authors known the facts as established in recent years, most of the hypotheses would never have been advanced. The carbon dioxide hypothesis is the only one which was framed with geologically rapid climatic alternations in mind. It certainly explains the facts of periodicity better than does any of its predecessors, but even so it does not account for the intimate way in which variations of all degrees from those of the weather up to glacial epochs seem to grade into one another.
According to our stellar hypothesis, occasional groups of glacial epochs would be expected to occur close together and to form long glacial periods. This is because many of the stars belong to groups or cl.u.s.ters in which the stars move in parallel paths. A good example is the cl.u.s.ter in the Hyades, where Boss has studied thirty-nine stars with special care.[120] The stars are grouped about a center about 130 light years from the sun. The stars themselves are scattered over an area about thirty light years in diameter. They average about the same distance apart as do those near the sun, but toward the center of the group they are somewhat closer together. The whole thirty-nine sweep forward in essentially parallel paths. Boss estimates that 800,000 years ago the cl.u.s.ter was only half as far from the sun as at present, but probably that was as near as it has been during recent geological times. All of the thirty-nine stars of this cl.u.s.ter, as Moulton[121] puts it, "are much greater in light-giving power than the sun. The luminosities of even the five smallest are from five to ten times that of the sun, while the largest are one hundred times greater in light-giving power than our own luminary. Their ma.s.ses are probably much greater than that of the sun." If the sun were to pa.s.s through such a cl.u.s.ter, first one star and then another might come so near as to cause a profound disturbance in the sun"s atmosphere.
3. Another important point upon which a glacial hypothesis may come to grief is the length of the periods or rather of the epochs which compose the periods. During the last or Pleistocene glacial period the evidence in America and Europe indicates that the inter-glacial epochs varied in length and that the later ones were shorter than the earlier. Chamberlin and Salisbury, from a comparison of various authorities, estimate that the intervals from one glacial epoch to another form a declining series, which may be roughly expressed as follows: 16-8-4-2-1, where unity is the interval from the climax of the late Wisconsin, or last glacial epoch, to the present. Most authorities estimate the culmination of the late Wisconsin glaciation as twenty or thirty thousand years ago. Penck estimates the length of the last inter-glacial period as 60,000 years and the preceding one as 240,000.[122] R. T. Chamberlin, as already stated, finds that the consensus of opinion is that inter-glacial epochs have averaged five times as long as glacial epochs. The actual duration of the various glaciations probably did not vary in so great a ratio as did the intervals from one glaciation to another. The main point, however, is the irregularity of the various periods.
The relation of the stellar electrical hypothesis to the length of glacial epochs may be estimated from column C, in Table 5. There we see that the distances at which a star might possibly disturb the sun enough to cause glaciation range all the way from 120 billion miles in the case of a small star like the sun, to 3200 billion in the case of Betelgeuse, while for double stars the figure may rise a hundred times higher. From this we can calculate how long it would take a star to pa.s.s from a point where its influence would first amount to a quarter of the a.s.sumed maximum to a similar point on the other side of the sun. In making these calculations we will a.s.sume that the relative rate at which the star and the sun approach each other is about twenty-two miles per second, or 700 million miles per year, which is the average rate of motion of all the known stars. According to the distances in Table 5 this gives a range from about 500 years up to about 10,000, which might rise to a million in the case of double stars. Of course the time might be relatively short if the sun and a rapidly moving star were approaching one another almost directly, or extremely long if the sun and the star were moving in almost the same direction and at somewhat similar rates,--a condition more common than the other. Here, as in so many other cases, the essential point is that the figures which we thus obtain seem to be of the right order of magnitude.
4. Post-glacial climatic stages are so well known that in Europe they have definite names. Their sequence has already been discussed in Chapter XII. Fossils found in the peat bogs of Denmark and Scandinavia, for example, prove that since the final disappearance of the continental ice cap at the close of the Wisconsin there has been at least one period when the climate of Europe was distinctly milder than now. Directly overlying the sheets of glacial drift laid down by the ice there is a flora corresponding to that of the present tundras. Next come remains of a forest vegetation dominated by birches and poplars, showing that the climate was growing a little warmer. Third, there follow evidences of a still more favorable climate in the form of a forest dominated by pines; fourth, one where oak predominates; and fifth, a flora similar to that of the Black Forest of Germany, indicating that in Scandinavia the temperature was then decidedly higher than today. This fifth flora has retreated southward once more, having been driven back to its present lat.i.tude by a slight recurrence of a cool stormy climate.[123] In central Asia evidence of post-glacial stages is found not only in five distinct moraines but in a corresponding series of elevated strands surrounding salt lakes and of river terraces in non-glaciated arid regions.[124]
In historic as well as prehistoric times, as we have already seen, there have been climatic fluctuations. For instance, the twelfth or thirteenth century B. C. appears to have been almost as mild as now, as does the seventh century B. C. On the other hand about 1000 B. C., at the time of Christ, and in the fourteenth century there were times of relative severity. Thus it appears that both on a large and on a small scale pulsations of climate are the rule. Any hypothesis of climatic changes must satisfy the periods of these pulsations. These conditions furnish a problem which makes difficulty for almost all hypotheses of climatic change. According to the present hypothesis, earth movements such as are discussed in Chapter XII may cooperate with two astronomical factors.
One is the constant change in the positions of the stars, a change which we have already called kaleidoscopic, and the other is the fact that a large proportion of the stars are double or multiple. When one star in a group approaches the sun closely enough to cause a great solar disturbance, numerous others may approach or recede and have a minor effect. Thus, whenever the sun is near groups of stars we should expect that the earth would show many minor climatic pulsations and stages which might or might not be connected with glaciation. The historic pulsations shown in the curve of tree growth in California, Fig. 4, are the sort of changes that would be expected if movements of the stars have an effect on the solar atmosphere.
Not only are fully a third of all the visible stars double, as we have already seen, but at least a tenth of these are known to be triple or multiple. In many of the double stars the two bodies are close together and revolve so rapidly that whatever periodicity they might create in the sun"s atmosphere would be very short. In the triplets, however, the third star is ordinarily at least ten times as far from the other two as they are from each other, and its period of rotation sometimes runs into hundreds or thousands of years. An actual multiple star in the constellation Polaris will serve as an example. The main star is believed by Jeans to consist of two parts which are almost in contact and whirl around each other with extraordinary speed in four days. If this is true they must keep each other"s atmospheres in a state of intense commotion. Much farther away a third star revolves around this pair in twelve years. At a much greater distance a fourth star revolves around the common center of gravity of itself and the other three in a period which may be 20,000 years. Still more complicated cases probably exist. Suppose such a system were to traverse a path where it would exert a perceptible influence on the sun for thirty or forty thousand years. The varying movements of its members would produce an intricate series of cycles which might show all sorts of major and minor variations in length and intensity. Thus the varied and irregular stages of glaciation and the pulsations of historic times might be accounted for on the hypothesis of the proximity of the sun to a multiple star, as well as on that of the less p.r.o.nounced approach and recession of a number of stars. In addition to all this, an almost infinitely complex series of climatic changes of long and short duration might arise if the sun pa.s.sed through a nebula.
5. We have seen in Chapter VIII that the contrast between the somewhat severe climate of the present and the generally mild climate of the past is one of the great geological problems. The glacial period is not a thing of the distant past. Geologists generally recognize that it is still with us. Greenland and Antarctica are both shrouded in ice sheets in lat.i.tudes where fossil floras prove that at other periods the climate was as mild as in England or even New Zealand. The present glaciated regions, be it noted, are on the polar borders of the world"s two most stormy oceanic areas, just where ice would be expected to last longest according to the solar cyclonic hypothesis. In contrast with the semi-glacial conditions of the present, the last inter-glacial epoch was so mild that not only men but elephants and hippopotamuses flourished in central Europe, while at earlier times in the middle of long eras, such as the Paleozoic and Mesozoic, corals, cycads, and tree ferns flourished within the Arctic circle.
If the electro-stellar hypothesis of solar disturbances proves well founded, it may explain these peculiarities. Periods of mild climate would represent a return of the sun and the earth to their normal conditions of quiet. At such times the atmosphere of the sun is a.s.sumed to be little disturbed by sunspots, faculae, prominences, and other allied evidences of movements; and the rice-grain structure is perhaps the most prominent of the solar markings. The earth at such times is supposed to be correspondingly free from cyclonic storms. Its winds are then largely of the purely planetary type, such as trade winds and westerlies. Its rainfall also is largely planetary rather than cyclonic.
It falls in places such as the heat equator where the air rises under the influence of heat, or on the windward slopes of mountains, or in regions where warm winds blow from the ocean over cold lands.
According to the electro-stellar hypothesis, the conditions which prevailed during hundreds of millions of years of mild climate mean merely that the solar system was then in parts of the heavens where stars--especially double stars--were rare or small, and electrical disturbances correspondingly weak. Today, on the other hand, the sun is fairly near a number of stars, many of which are large doubles. Hence it is supposed to be disturbed, although not so much as at the height of the last glacial epoch.
After the preceding parts of this book had been written, the a.s.sistance of Dr. Schlesinger made it possible to test the electro-stellar hypothesis by comparing actual astronomical dates with the dates of climatic or solar phenomena. In order to make this possible, Dr.
Schlesinger and his a.s.sistants have prepared Table 6, giving the position, magnitude, and motions of the thirty-eight nearest stars, and especially the date at which each was nearest the sun. In column 10 where the dates are given, a minus sign indicates the past and a plus sign the future. Dr. Shapley has kindly added column 12, giving the absolute magnitudes of the stars, that of the sun being 4.8, and column 13, showing their luminosity or absolute radiation, that of the sun being unity. Finally, column 14 shows the effective radiation received by the sun from each star when the star is at a minimum distance. Unity in this case is the effect of a star like the sun at a distance of one light year.
It is well known that radiation of all kinds, including light, heat, and electrical emissions, varies in direct proportion to the exposed surface, that is, as the square of the radius of a sphere, and inversely as the square of the distance. From black bodies, as we have seen, the total radiation varies as the fourth power of the absolute temperature.
It is not certain that either light or electrical emissions from incandescent bodies vary in quite this same proportion, nor is it yet certain whether luminous and electrical emissions vary exactly together.
Nevertheless they are closely related. Since the light coming from each star is accurately measured, while no information is available as to electrical emissions, we have followed Dr. Shapley"s suggestion and used the luminosity of the stars as the best available measure of total radiation. This is presumably an approximate measure of electrical activity, provided some allowance be made for disturbances by outside bodies such as companion stars. Hence the inclusion of column 14.
TABLE 6
THIRTY-EIGHT STARS HAVING LARGEST KNOWN PARALLAXES
Star Code 1 Groombr. 34 2 ++[Greek: e] Ca.s.siop.
3 4 ++[Greek: k] Tucanae 5 [Greek: t] Ceti 6 [Greek: d]_2 Eridani 7 ++[Greek: e] Eridani 8 ++40(0)^2 Eridani 9 Cordoba Z. 243 10 Weisse 592 11 ++[Greek: a] Can. Maj. (Sirius) 12 ++[Greek: a] Can. Min. (Procyon) 13 ++Fedorenko 1457-8 14 Groombr. 1618 15 Weisse 234 16 Lalande 21185 17 Lalande 21258 18 19 Lalande 25372 20 ++[Greek: a] Centauri 21 ++[Greek: x] Bootes 22 ++Lalande 27173 23 Weisse 1259 24 Lacaille 7194 25 ++[Greek: b] 416 26 Argel -0.17415-6 27 Barnard"s star 28 ++70p Ophiuchi 29 ++[Greek: S] 2398 30 [Greek: s] Draconis 31 ++[Greek: a] Aquilae (Altair) 32 ++61 Cygni 33 Lacaille 8760 34 [Greek: e] Indi 35 ++Kruger 60 36 Lacaille 9352 37 Lalande 46650 38 C. G. A. 32416
(++ Double star.)
(1) (2) (3) (4) (5) (6) Right Declination Visual Spectrum Proper Radial Star Ascension [Greek: d] Mag. m Motion Velocity code [Greek: a] 1900 km. per 1900 sec.
------------------------------------------------------------------ 1 0^h 12^m.7 +4327" 8.1 Ma 2".89 + 3 2 43 .0 +57 17 3.6 F8 1 .24 + 10 3 43 .9 +4 55 12.3 F0 3 .01 .....
4 1 12 .4 -69 24 5.0 F8 .39 + 12 5 39 .4 -16 28 3.6 K0 1 .92 - 16 ------------------------------------------------------------------ 6 3 15 .9 -43 27 4.3 G5 3 .16 + 87 7 28 .2 - 9 48 3.8 K0 .97 + 16 8 4 10 .7 - 7 49 4.5 G5 4 .08 - 42 9 5 7 .7 -44 59 9.2 K2 8 .75 +242 10 26 .4 - 3 42 8.8 K2 2 .22 .....
------------------------------------------------------------------ 11 6 40 .7 -16 35 -1.6 A0 1 .32 - 8 12 7 34 .1 + 5 29 0.5 F5 1 .24 - 4 13 9 7 .6 +53 7 7.9 Ma 1 .68 + 10 14 10 5 .3 +49 58 6.8 K5p 1 .45 - 30 15 14 .2 +20 22 9.0 ... .49 .....
------------------------------------------------------------------ 16 57 .9 +36 38 7.6 Mb 4 .78 - 87 17 11 0 .5 +44 2 8.5 K5 4 .52 + 65 18 12 .0 -57 2 12.0 ... 2 .69 .....
19 13 40 .7 +15 26 8.5 K5 2 .30 .....
20 14 32 .8 -60 25 0.2 G 3 .68 + 22 ------------------------------------------------------------------ 21 14 46 .8 +19 31 4.6 K5p .17 + 4 22 51 .6 -20 58 5.8 Kp 1 .96 + 20 23 16 41 .4 +33 41 8.4 ... .37 .....
24 17 11 .5 -46 32 5.7 K .97 .....
25 12 .1 -34 53 5.9 K5 1 .19 - 4 ------------------------------------------------------------------ 26 37 .0 +68 26 9.1 K 1 .33 .....
27 52 .9 + 4 25 9.7 Mb 10 .30 - 80 28 18 0 .4 + 2 31 4.3 K 1 .13 .....
29 41 .7 +59 29 8.8 K 2 .31 .....
30 19 32 .5 +69 29 4.8 G5 1 .84 + 26 ------------------------------------------------------------------ 31 45 .9 + 8 36 1.2 A5 .66 - 33 32 21 2 .4 +38 15 5.6 K5 5 .20 - 64 33 11 .4 -39 15 6.6 G 3 .53 + 13 34 55 .7 -57 12 4.8 K5 4 .70 - 39 35 22 24 .4 +57 12 9.2 ... .87 .....
------------------------------------------------------------------ 36 59 .4 -36 26 7.1 K 6 .90 + 12 37 23 44 .0 + 1 52 8.7 Ma 1 .39 .....
38 59 .5 -37 51 8.2 G 6 .05 + 26
(7) (9) (11) (13) (14) Present Minimum Magnitude Luminosity Effective Parallax Distance at Min. Dist.radiation [Greek: p] Light Yrs.atminimum(8)(10)(12)distance StarMaximumTime ofAbsolutefrom sun CodeParallaxMinimumMagnitudeDistance----------------------------------------------------------------------- 1 ".28 ".28 11.6 -4000 8.1 10.3 0.0063 0.000051 2 .18 .19 17.1 -47000 3.5 4.9 0.91 0.003110 3 .24 .... .... ...... .... 14.2 0.00017 ........
4 .16 .23 14.2 -264000 4.2 6.0 0.33 0.001610 5 .32 .37 8.8 +46000 3.3 6.1 0.30 0.003840 ----------------------------------------------------------------------- 6 .16 .22 14.8 -33000 3.6 5.3 0.63 0.002960 7 .31 .46 7.1 -106000 3.0 6.3 0.25 0.004970 8 .21 .23 14.2 +19000 4.3 6.1 0.30 0.001470 9 .32 .68 4.8 -10000 7.6 11.7 0.0017 0.000074 10 .17 .... .... ...... .... 9.9 0.009 ........