of a zone of heavier precipitation must have had the effect of joining up isolated patches of woodland and making con- tinuous forest out of them; or where a diminution of rain- fall would have the effect of making the forest growth still more patchy or causing it almost to disappear for a time. The effect upon the grasses of the belt of pastures must be similar. There must have been a thinning out of pastures at the upper limit of the zone of pasture as we see it today and an extension of the pastures at the lower limit of the zone. The increasing cold of the glacial period had the effect of lowering the snow line and the successive climatic zones along with it. The belt of pastures must have been brought down to the summits of mountains that now have no pastures at all of the sort that prevail in the belt of high mountain country of which we speak. Again, there must have been mountains whose summits were covered with pastures before the glacial period and at the present time and that were free from pastures during the glacial period. These circumstances are suggested in Figure 115, the upper and lower levels of the zone of pasture being in critical relation to mountains or mountain ranges, as we may suppose them to be, of three different heights. In addition to the mountain pastures there would be, of course, a wide deployment of the plains pastures to correspond with the wetter climate. In addition to the raising and lowering of the zones of grass and woodland upon the mountain flanks, there would be an absolute increase in grass and woodland on account of this heavier precipitation. The climate of the glacial period was clearly one characterized by more moisture. The ground water and the level of lakes and streams would surely feel the effect of such an increase in available moisture.
Vegetational Changes as an Index
No one has yet applied to northwestern Argentina the results of field investigations like those made by Clements in our Southwest.[1] From a comparative study of vegetation
- ↑ F. E. Clements: The Original Grassland of Mohave and Colorado Deserts, Report on Investigations in Ecology, Year Book Carnegie Instn. No. 21 for 922, pp. 350-351.
