As before remarked, all the regions which have been subjected to glaciation are still each year brought temporarily into the glacial state. This fact serves to show us that the changes necessary to produce great ice sheets are not necessarily of a startling nature, however great the consequences may be. Assuming, then, that relatively slight alterations of climate may cause the ice sheet to come and go, we may say that all the influences which have been suggested by the students of glaciation, and various other slighter causes which can not be here noted, may have co-operated to produce the peculiar result. In this equation geographic change has affected the course of the ocean currents, and has probably been the most influential, or at least the commonest, cause to which we must attribute the extension of ice sheets. Next, alterations of the solar heat may be looked to as a change-bringing action; unfortunately, however, we have no direct evidence that this is an efficient cause. Thirdly, the variations in the eccentricity of the earth's orbit, combined with the precession of the equinoxes and the rotation of the apsides, may be regarded as operative. The last of all, changes in the constitution of the atmosphere, have to be taken into account. To these must be added, as before remarked, many less important actions which influence this marvellously delicate machine, the work of which is expressed in the phenomena assembled under the name of climate. Evidence is slowly accumulating which serves to show that glacial periods of greater or less importance have been of frequent occurrence at all stages in the history of the earth of which we have a distinct record. As these accidents write their history upon the ground alone, and in a way impermanently, it is difficult to trace the ice times of ancient geological periods. The scratches on the bed rocks, and the accumulations of detritus formed as the ice disappeared, have alike been worn away by the agents of decay. Nevertheless, we can trace here and there in the older strata accumulations of pebbly matter often containing large boulders, which clearly were shaped and brought together by glacial action. These are found in some instances far south of the region occupied by the glaciers during the last ice epoch. They occur in rocks of the Cambrian or Silurian age in eastern Tennessee and western North Carolina; they are also found in India beyond the limits to which glaciers have attained in modern times.