488 GEOLOGY. 



Mixing of relics. — Not only was such an ideal sjmimetry in the suc- 

 cession of faunas and floras too delicate to be often perfectly preserved, 

 but it was easily subject to mutilation and mixture. Relics which 

 were deposited in the first stages of retreat were liable to be washed 

 out by the succeeding drainage and commingled with the deposits 

 of a later stage. So also, as these interglacial beds were loose deposits 

 and more or less exposed at the surface, they were subject, at vari- 

 ous later times, to various kinds of disturbance, as by the burrowing 

 of animals, the overturning of trees, the filling of root-holes, and the 

 various incidental disturbances which affect loose superficial deposits. 

 There were also normal shiftings of fluvial material, the reworking 

 of river-bottoms and terraces, the cutting and filling of gullies, the 

 creeping and sliding on declivities, the inevitable slope-wash, and simi- 

 lar surface disturbances. Unusual circumspection is therefore requisite 

 in observing and interpreting the life relics found in this class of deposits. 



Real intermingling of northern and southern species. — Besides the 

 post-depositional mixing of forms that were originally separate, there 

 was undoubtedly a true intermingling of northern and southern species 

 while living, for the migrations could not well keep even pace with 

 the climatic variations. Plants necessarily lingered until the invading 

 climate destroyed them. The species migrated by the accidental 

 transportation of their seeds, but the individual plants had no such 

 power of migration, and they, and the offspring of such seed as they 

 planted beneath and about them, remained until destroyed. Under 

 these conditions, the advance forms of each shifting zone must inevi- 

 tably have overtaken and mingled with the lingering forms of the 

 adjacent zone, and these must have been subject to burial and fossiliza- 

 tion together. This also serves to perplex interpretation. 



Even in the case of animal species whose facilities for migration 

 are freer, the literature of the subject contains puzzling statements 

 of strange associations. In the caves of Britain, the relics of the arctic 

 musk-ox are said to be found closely associated with those of the hippo- 

 potamus; in the caves of France, the relics of the reindeer with those 

 of the lion; in the caves of Belgium, the auroch and the Alpine chamois 

 with the sub-tropical hyena. 



Cave deposits. — A special phase of record, and also of the mixing 

 of relics, is found in the cave deposits of the period. Caves were un- 

 doubtedly the resorts of land animals in the Tertiary and earlier periods, 



