found now in fairly humid regions, for example, the Pennsylvanian and Permian deposits of Kansas and Oklahoma, were probably laid down in times of local aridity due to the cutting off of moisture-bearing winds by the mountains of Llanoria in Louisiana and Texas. Hence such deposits do not necessarily indicate periods of widespread and profound aridity.
When the causes of ancient glaciation were first considered by geologists, about the middle of the nineteenth century, it was usually assumed that the glaciated areas had been elevated to great heights, and thus rendered cold enough to permit the accumulation of glaciers. The many glaciers occurring in the Alps of central Europe where glaciology arose doubtless suggested this explanation. However, it is now known that most of the ancient glaciation was not of the alpine type, and there is adequate proof that the glacial periods cannot be explained as due directly and solely to uplift. Nevertheless, upheavals of the lands are among the most important factors in controlling climate, and variations in the height of the lands have doubtless assisted in producing climate oscillations, especially those of long duration. Moreover, the progressive increase in the height of the lands has presumably played a part in fostering local and zonal diversity in contrast with the relative uniformity of earlier geological times.
IV. The contraction of the earth has been accompanied by volcanic activity as well as by changes in the extent, distribution, and altitude of the lands. The probable part played by volcanic dust as a contributory factor in producing short sudden climatic variations has already been discussed. There is, however, another though probably less important respect in which volcanic activity may have had at least a slight climatic significance. The oldest
known rocks, those of the Archean era, contain so much igneous matter that many students have assumed that they show that the entire earth was once liquid. It is now considered that they merely indicate igneous activity of great magnitude. In the later part of Proterozoic time, during the second quarter of the earth's history according to Schuchert's estimate, there were again vast outflowings of lava. In the Lake Superior district, for example, a thickness of more than a mile accumulated over a large area, and lavas are common in many areas where rocks of this age are known. The next quarter of the earth's history elapsed without any correspondingly great outflows so far as is known, though several lesser ones occurred. Toward the end of the last quarter, and hence quite recently from the geological standpoint, another period of outflows, perhaps as noteworthy as that of the Proterozoic, occurred in the Cretaceous and Tertiary.
The climatic effects of such extensive lava flows would be essentially as follows: In the first place so long as the lavas were hot they would set up a local system of convection with inflowing winds. This would interfere at least a little with the general winds of the area. Again, where the lava flowed out into water, or where rain fell upon hot lava, there would be rapid evaporation which would increase the rainfall. Then after the lava had cooled, it would still influence climate a trifle in so far as its color was notably darker or lighter than that of the average surface. Dark surfaces absorb solar heat and become relatively warm when the sun shines upon them. Dark objects likewise radiate heat more rapidly than light-colored objects. Hence they cool more rapidly at night, and in the winter. As most lavas are relatively dark they increase the average diurnal range of temperature.
Hence even after they are cool they increase the climatic diversity of the land.
The amount of heat given to the atmosphere by an extensive lava flow, though large according to human standards, is small compared with the amount received from the sun by a like area, except during the first few weeks or months before the lava has formed a thick crust. Furthermore, probably only a small fraction of any large series of flows occurred in a given century or millennium. Moreover, even the largest lava flows covered an area of only a few hundredths of one per cent of the earth's surface. Nevertheless, the conditions which modify climate are so complicated that it would be rash to state that this amount of additional heat has been of no climatic significance. Like the proverbial "straw that broke the camel's back," the changes it would surely produce in local convection, atmospheric pressure, and the direction of the wind may have helped to shift the paths of storms and to produce other complications which were of appreciable climatic significance.
V. The last point which we shall consider in connection with the effect of the earth's interior upon climate is internal heat. The heat given off by lavas is merely a small part of that which is emitted by the earth as a whole. In the earliest part of geological history enough heat may have escaped from the interior of the earth to exert a profound influence on the climate. Knowlton,[94] as we have seen, has recently built up an elaborate theory on this assumption. At present, however, accurate measurements show that the escape of heat is so slight that it has no appreciable influence except in a few volcanic
areas. It is estimated to raise the average temperature of the earth's surface less than 0.1°C.[95]
In order to contribute enough heat to raise the surface temperature 1°C., the temperature gradient from the interior of the earth to the surface would need to be ten times as great as now, for the rate of conduction varies directly with the gradient. If the gradient were ten times as great as now, the rocks at a depth of two and one-half miles would be so hot as to be almost liquid according to Barrell's[96] estimates. The thick strata of unmetamorphosed Paleozoic rocks indicate that such high temperatures have not prevailed at such slight depths since the Proterozoic. Furthermore, the fact that the climate was cold enough to permit glaciation early in the Proterozoic era and at from one to three other times before the opening of the Paleozoic suggests that the rate of escape of heat was not rapid even in the first half of the earth's recorded history. Yet even if the general escape of heat has never been large since the beginning of the better-known part of geological history, it was presumably greater in early times than at present.