As a preliminary to this study it may be well to note that the coincidence between movements of the earth's crust and climatic changes is not so absolute as is sometimes supposed. For example, the profound crustal changes at the end of the Mesozoic were not accompanied by widespread glaciation so far as is yet known, although
the temperature appears to have been lowered. Nor was the violent volcanic and diastrophic activity in the Miocene associated with extreme climates. Indeed, there appears to have been little contrast from zone to zone, for figs, bread fruit trees, tree ferns, and other plants of low latitudes grew in Greenland. Nevertheless, both at the end of the Mesozoic and in the Miocene the climate may possibly have been severe for a time, although the record is lost. On the other hand, Kirk's recent discovery of glacial till in Alaska between beds carrying an undoubted Middle Silurian fauna indicates glaciation at a time when there was little movement of the crust so far as yet appears.[129] Thus we conclude that while climatic changes and crustal movements usually occur together, they may occur separately.
According to the solar-cyclonic hypothesis such a condition is to be expected. If the sun were especially active when the terrestrial conditions prohibited glaciation, changes of climate would still occur, but they would be milder than under other circumstances, and would leave little record in the rocks. Or there might be glaciation in high latitudes, such as that of southern Alaska in the Middle Silurian, and none elsewhere. On the other hand, when the sun was so inactive that no great storminess occurred, the upheaval of continents and the building of mountains might go on without the formation of ice sheets, as apparently happened at the end of the Mesozoic. The lack of absolute coincidence between glaciation and periods of widespread emergence of the lands is evident even today, for there is no reason to suppose that the lands are notably lower or less extensive now than they were during the Pleistocene glaciation. In fact, there is much evidence that many areas have risen
since that time. Yet glaciation is now far less extensive than in the Pleistocene. Any attempt to explain this difference on the basis of terrestrial changes is extremely difficult, for the shape and altitude of continents and mountains have not changed much in twenty or thirty thousand years. Yet the present moderately mild epoch, like the puzzling inter-glacial epochs of earlier times, is easily explicable on the assumption that the sun's atmosphere may sometimes vary in harmony with crustal activity, but does not necessarily do so at all times.
Turning now to the main problem of how climatic changes may be connected with movements of the earth's crust, let us follow our usual method and examine what is happening today. Let us first inquire whether earthquakes, which are one of the chief evidences that crustal movements are actually taking place in our own times, show any connection with sunspots. In order to test this, we have compared Milne's Catalogue of Destructive Earthquakes from 1800 to 1899, with Wolf's sunspot numbers for the same period month by month. The earthquake catalogue, as its compiler describes it, "is an attempt to give a list of earthquakes which have announced changes of geological importance in the earth's crust; movements which have probably resulted in the creation or the extension of a line of fault, the vibrations accompanying which could, with proper instruments, have been recorded over a continent or the whole surface of our world. Small earthquakes have been excluded, while the number of large earthquakes both for ancient and modern times has been extended. As an illustration of exclusion, I may mention that between 1800 and 1808, which are years taken at random, I find in Mallet's catalogue 407 entries. Only thirty-seven of these, which were accompanied by structural damage, have been retained.
Other catalogues such as those of Perry and Fuchs have been treated similarly."[130]
If the earthquakes in such a carefully selected list bear a distinct relation to sunspots, it is at least possible and perhaps probable that a similar relation may exist between solar activity and geological changes in the earth's crust. The result of the comparison of earthquakes and sunspots is shown in Table 7. The first column gives the sunspot numbers; the second, the number of months that had the respective spot numbers during the century from 1800 to 1899. Column C shows the total number of earthquakes during the months having any particular degree of spottedness; while D, which is the significant column, gives the average number of destructive earthquakes per month under each of the six conditions of solar spottedness.
| [TABLE 7] | |||||
|---|---|---|---|---|---|
| DESTRUCTIVE EARTHQUAKES FROM 1800 TO1899 COMPARED WITH SUNSPOTS | |||||
| A | B | C | D | E | F |
| Sunspot numbers | Number of months per Wolf's Table | Number of earthquakes | Average number of earthquakes per month | Number of earthquakes in succeeding month | Average number of earthquakes in succeeding month |
| 0-15 | 344 | 522 | 1.52 | 512 | 1.49 |
| 15-30 | 194 | 306 | 1.58 | 310 | 1.60 |
| 30-50 | 237 | 433 | 1.83 | 439 | 1.85 |
| 50-70 | 195 | 402 | 2.06 | 390 | 2.00 |
| 70-100 | 135 | 286 | 2.12 | 310 | 2.30 |
| over 100 | 95 | 218 | 2.30 | 175 | 1.84 |
The regularity of column D is so great as to make it almost certain that we are here dealing with a real relationship. Column F, which shows the average number of earthquakes in the month succeeding any given condition of the sun, is still more regular except for the last entry.
The chance that six numbers taken at random will arrange themselves in any given order is one in 720. In other words, there is one chance in 720 that the regularity of column D is accidental. But column F is as regular as column D except for the last entry. If columns D and E were independent there would be one chance in about 500,000 that the six numbers in both columns would fall in the same order, and one chance in 14,400 that five numbers in each would fall in the same order. But the two columns are somewhat related, for although the after-shocks of a great earthquake are never included in Milne's table, a world-shaking earthquake in one region during a given month probably creates conditions that favor similar earthquakes elsewhere during the next month. Hence the probability that we are dealing with a purely accidental arrangement in Table 7 is less than one in 14,400 and greater than one in 500,000. It may be one in 20,000 or 100,000. In any event it is so slight that there is high probability that directly or indirectly sunspots and earthquakes are somehow connected.