And now, let us consider briefly the characters of the rocks of the crust, to see if they throw any light upon this question. The earliest sediments of which we have any certain knowledge resemble in a striking manner those formed at the present day, and they seem to have been formed under very much the same conditions, though further work may show that there were somewhat different conditions which did produce definite differences in the characters of the earlier strata[127]. Our knowledge of earth-movement and vulcanicity which took place in past times is still too small to enable us to draw any certain conclusions connected with the subject under discussion from it. Perhaps the most suggestive indication of one set of conditions having been generally similar in those early periods of which we have definite records amongst the rocks is furnished by study of past climate. If we accept the nebular hypothesis as a starting point, we must admit that in the early stages of the earth's history the temperature of the surface, which would then be largely dependent upon the amount of heat given out from the earth itself as well as upon that received from the sun, must have been much higher than it is at the present day, and indeed the mere diminution of the amount of heat received from the sun would probably be sufficient to account for a very marked lowering of the temperature. Besides this change of temperature, resulting in gradual lowering of temperature over the whole earth's surface, we have other changes dependent upon different conditions, as proved by the fact, that there have been alternations of glacial and genial periods. If the general temperature had been very high in the early periods of which we have actual records, the oscillations would not be sufficient to produce a lowering of temperature sufficient to cause glacial periods, whereas if it had not been appreciably higher than now, glacial periods might be produced. This may be represented diagrammatically. [Footnote 127: On this matter see Teall, J. J. H., 'Presidential Address to Section C,' _Report of the British Association_, 1893.]