IV.  UNCLASSIFIED POST-PLIOCENE DEPOSITS.—­Apart from any of the afore mentioned deposits, there occur other accumulations—­sometimes superficial, sometimes in caves—­which are found in regions where a “Glacial period” has not been fully demonstrated, or where such did not take place; and which, therefore, are not amenable to the above classification.  The most important of these are known to occur in South America and Australia; and though their numerous extinct Mammalia place their reference to the Post-Pliocene period beyond doubt, their relations to the glacial period and its deposits in the northern hemisphere have not been precisely determined.

CHAPTER XXII.

THE POST-PLIOCENE PERIOD—­Continued.

As regards the life of the Post-Pliocene period, we have, in the first place, to notice the effect produced throughout the northern hemisphere by the gradual supervention of the Glacial period.  Previous to this the climate must have been temperate or warm-temperate; but as the cold gradually came on, two results were produced as regards the living beings of the area thus affected.  In the first place, all those Mammals which, like the Mammoth, the Woolly Rhinoceros, the Lion, the Hyaena, and the Hippopotamus, require, at any rate, moderately warm conditions, would be forced to migrate southwards to regions not affected by the new state of things.  In the second place, Mammals previously inhabiting higher latitudes, such as the Reindeer, the Musk-ox, and the Lemming, would be enabled by the increasing cold to migrate southwards, and to invade provinces previously occupied by the Elephant and the Rhinoceros.  A precisely similar, but more slowly-executed process, must have taken place in the sea, the northern Mollusca moving southwards as the arctic conditions of the Glacial period became established, whilst the forms proper to temperate seas receded.  As regards the readily locomotive Mammals, also, it is probable that this process was carried on repeatedly in a partial manner, the southern and northern forms alternately fluctuating backwards and forwards over the same area, in accordance with the fluctuations of temperature which have been shown by Mr James Geikie to have characterised the Glacial period as a whole.  We can thus readily account for the intermixture which is sometimes found of northern and southern types of Mammalia in the same deposits, or in deposits apparently synchronous, and within a single district.  Lastly, at the final close of the arctic cold of the Glacial period, and the re-establishment of temperate conditions over the northern hemisphere, a reversal of the original process took place—­the northern Mammals retiring within their ancient limits, and the southern forms pressing northwards and reoccupying their original domains.

The Invertebrate animals of the Post-Pliocene deposits require no further mention—­all the known forms, except a few of the shells in the lowest beds of the formation, being identical with species now in existence upon the globe.  The only point of importance in this connection has been previously noticed—­namely, that in the true Glacial deposits themselves a considerable number of the shells belong to northern or Arctic types.