The age of stone is subdivided by Sir John Lubbock into the neolithic periods, or that in which polished stone was the only material used for cutting, and the palæolithic, in which mankind had not learnt to grind and polish his implements. The latter belongs to the pleistocene, or quaternary period, since the palæolithic implements are found in association with the remains of the animals characteristic of that age. The prehistoric caves, therefore, may be divided into three classes if the archæological method of analysis be employed: 1, into those containing evidence of the use of iron; 2, those containing proof of the knowledge of bronze; 3, and lastly, those in which traces of polished stone weapons have been discovered unassociated with metals. By the animal remains which they contain they may be distinguished from those of the pleistocene age, both by the absence, as well as the presence of certain species which have been enumerated. From the archæological point of view, two out of the four ages are still represented. Stone is, at the present time, the only material used in the more remote regions of Australia, although it is fast being replaced by iron, which has superseded bronze, and is spreading rapidly over the whole earth. The group of historic caves described in the preceding chapter may be said to belong to the iron age, that is to say, to that later portion of it in which the events are recorded in history. The traces of the occupation of caves by man in the iron and bronze ages are so extremely scarce, that it is certain that they were but rarely used as habitations. Man had sufficiently advanced in civilization in those times to construct artificial dwellings and tombs for himself, instead of using the natural shelters which were so very generally occupied in Europe by his ruder neolithic predecessors. _Cave of the Iron Age._