With reference to these points, science has as yet little information to give. Sir William Thomson has, indeed, indicated for the time since the earth's crust first began to form a period of between one and two hundred millions of years; but Professor Guthrie Tait, on the other hand, argues that ten or fifteen millions of years are probably sufficient,[65] and Lockyer has suggested an hypothesis of successive rekindlings of the solar heat which might give a more protracted time than that of Thomson. Some of the hypotheses of derivation current, but which are based rather on philosophical speculation than on scientific fact, would also require a longer time than that allowed by Thomson; and it is to be regretted that some geologists, by giving credence to such hypotheses of derivation, and by loose reasoning on the time required for the denudation and deposition of rocks, have been induced to commit themselves to very extravagant estimates as to geological time. On the whole, it is evident that only the most vague guesses can at present be based on the facts in our possession, though the whole time required has unquestionably been very great, the deposition of the series of stratified rocks probably requiring at least the greater part of the minimum time allowed by Thomson.[66] As to the cosmical nature of the periods, while some geologists appear to regard the whole of geological time as a continuous evolution without any breaks, it is evidently more in accordance with facts to hold that there have been cycles of repose and activity succeeding each other, and that these have been of different grades. In the succession of deposits it is plain that periods of depression and upheaval common to all the continental masses have succeeded each other at somewhat regular intervals, and that within these periods there have been alternations of colder and warmer climates. These, however, are not equal to the creative days of our record, for they are greatly more numerous. They are but the vastly protracted hours of these almost endless days. Beyond and above these there is another grade of geological period, marked not by mere gradual elevation and depression of the continental areas, but by vast crumplings of the earth's crust and enormous changes of level. Such a great movement unquestionably closed the Eozoic period of geology. Another of less magnitude occurred in what is termed the Permian age at the end of the Palaeozoic. A third terminated the Mesozoic age, and introduced the Tertiary or Kainozoic. Perhaps we should reckon the glacial age, though characterized by far less physical change than the others, as a fourth. The possible physical causes which have been suggested for such greater disturbances are the collapses of the crust in equatorial regions, which may be supposed to have resulted at long intervals of time, from the gradual retardation of the earth's rotation caused by the tides, or the similar collapses and other changes due to the