Necessity of caution. more We must use caution, however, not to attach a definite value or measurement to processes or agencies which by their very nature are variable, and therefore indefinite. The rate of river erosions, for instance, on which much reliance is sometimes placed, varies according to the quantity of water and rapidity of the current; and these may change from year to year, still from one century to another. Likewise, the growth of peat and accumulation of stalagmitic crusts, vary between wide limits in different localities, and from time to time in the same. place. Again, great changes of climate, and entire change of the types of vegetable and animal life, may be regarded as indicating extended periods of time, but we do not sufficiently understand the causes of these changes to make them the basis of definite calculations. Indeed, it is quite evident that such changes are far from uniform in their rates of progress. Our dependence. There are certain indications, however, that may be studied with more confidence. These are found chiefly in peat-bogs, cavedeposits, shell-heaps, and in remains of ancient habitations in Switzerland known as lakedwellings. But to facilitate our study let us lay out our work more definitely; considering first the geologi cal divisions of the Quaternary Age, and then the periods into which the era of man's existence is usually divided. The geologist commonly divides the Quaternary Age of the earth's history into three Periods of periods, the Glacial, Champlain, and Ter- the Quaterrace. For convenience in our discussion nary Age. we have added a fourth, the Present period corresponding to what Prof. B. F. Mudge, of Kansas, has styled the Delta period, in allusion to the deposits now forming at the mouths of great rivers, as the Mississippi, the Nile, and the Ganges.* Glacial period. In the first, or glacial period, all or the greater part of this continent, as far south as the Ohio River and the southern line of Pennsylvania, was covered with a great depth of ice, as shown by glacial scratches, and by erratic boulders scattered here and there over the country, at a wide distance from the beds in which. they originated. And in Europe the ice prevailed as far south as Northern Italy. This was, of course, a period of extreme cold. There could have been little, if any life, either animal or vegetable, in the higher latitudes, and man could have lived only along the skirts of the glacier, or after it had retreated. * The reader is here referred to the upper section of the Chart affixed to the Sixth Lecture. Page 123. The animal kingdom was represented by the Mammoth and Rhinoceros, which had survived from an earlier period, the Cave-bear and Hyena, and somewhat later by the Reindeer; all of which have been long extinct, save the Reindeer, which has migrated to a northern clime, following close, as would seem, upon the receding ice. Succeeding the Glacial, or "Great Ice" period, came the Champlain, marked by a lower Champlain general level of land, a consequent wider period. extent of sea and warmer climate. In this period the glaciers melted in the regions now covered by the temperate zones, retreated northward or toward mountain tops, leaving their vast accumulations of rocks, gravel, clay, sand, and the like, to which the geologist applies the general name of Diluvium or Drift. The animals of this period differed in a marked degree from those of the preceding, as the changed climate would lead us to expect, and included the huge sloth-like Megatherium, with a considerable number that still survive, as the lion, tiger, wild boar, ox, horse, and deer. There were monkies also in Asia and marsupials in Australia. Terrace period. Following the Champlain came the Terrace period, during which the land gradually rose again, the sea withdrew to its present limits, and the successive levels called. Terraces were formed along the rivers by the gradual withdrawal of the streams to narrow and deeper channels. Some fine examples of the terraces of this period may be observed at Walpole and Hanover, N. H., and elsewhere along the valley of the Connecticut. The climate again had undergone a change; was colder than that of the Champlain, but milder than that of the Glacial epoch. animals were similar to those that live to-day, and need not therefore be described or named. The Now, as to the bearing of these facts upon the subject we have in hand. There is no doubt of the existence of man from the earlier Champlain through the whole of the Terrace period, to the present. There is little question but that man lived in southerly latitudes in the latter part of the Glacial period. Beyond that we must proceed with extreme caution. But let us not anticipate. Having noted particularly the different geological periods into which the Quaternary Age of the earth is divided, let us mark next the periods. into which it is customary to divide the era of man's existence; observing that the two series are entirely independent of each other. Archæologists distinguish four differ- The Era of ent periods in the existence of the hu Man. man race, according to the degree of advancement in art, namely: 1. The Paleolithic, or Rough Stone Age. 2. The Neolithic, or Polished Stone Age. 3. The Bronze Age, and 4. The Iron Age. Some authorities recognize still other distinctions and mark other divisions. But the above is very simple and sufficiently exact for our purpose. The Rough Stone Age marks the rudest stage of man's existence; when arrows, knives Paleolithic and other implements of the chase and Age. for domestic use, were roughly shapen from hard stone, chiefly flint and argillite. The The The Polished Stone Age marks a period of some advancement upon the condition of Neolithic the former, when men had learned to Age. smooth and polish their implements; and they employed a greater variety of hard stones, including porphyry, greenstone, and occasionally ob sidian and jasper. The Bronze Age. The Bronze Age marks the early use of metals in the arts-not the earliest, certainly, for men must have used copper before they learned to mix it with tin, producing the alloy known as bronze. But where metal was used in the same way as stone, that is, without fusing and moulding, the people must be regarded as still in the Stone Age. The Iron Age marks the higher civilization, |