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326 of the subarctic steppes which are of most importance from our present point of view.

1. Steppes, like tundras, are not exclusively plains. They include rocky uplands and hills, and are traversed in many places by streams and rivers.

2. Vast expanses are clothed with grasses, while others are more or less sterile and bare. Oases of forests are not infrequently present.

3. The most characteristic animals are jerboas, pouched marmots, bobacs, and others—the mammalian fauna being more varied than that of the tundras.

4. Many animals properly belonging to forest lands and to mountains frequent the steppes.

5. The seasons are strongly contrasted, and the whole region is exposed to dust storms in summer, and to snowstorms in winter.

With these facts relating to existing tundras and steppes kept in view, let ns now examine the evidence adduced by geologists to show that tundra and steppe conditions have successively prevailed in Middle Europe.

One of the most remarkable superficial deposits of central and west- central Europe is that which is known under the general term of löss. Typically it is a fine-grained, yellowish, calcareous, sandy loam—consisting very largely of minute grains of quartz, with some admixture of argillaceous and calcareous matter. Upon the whole the quartz grains are well rounded, although often enough they are sharply angular. Frequently the accumulation shows a porous structure, and is penetrated by long, approximately vertical root-like tubes or canals, lined with calcareous matter, which cause the deposit to cleave or divide in vertical planes. Hence it usually forms more or less upright bluffs upon the margins of streams or rivers which intersect it. It is usually unstratified, except now and again toward the bottom of the deposit, where intercalated layers, and even sometimes thick beds of sand, make their appearance. The loss is essentially a deposit of the low grounds, and is well developed in the broad river valleys of western and central Europe, as in those of the Seine, the Garonne, the Rhone, the Maas, the Moselle, the Rhine and its tributaries, the Danube and many of its affluents, such as the Drave, the Save, the Morava, and the Theiss. It also extends as a narrow belt along the southern margin of the great plains of North Germany. It is in southern and southeastern Russia, however, where it attains its widest development, covering as it does an immense tract, stretching west and east between the valleys of the Pruth and the Volga. Throughout this vast region it is usually very dark in color, forming what is known as the black earth.

Without at present going into the question as to the origin of the materials of which the loss is composed, it is obvious enough that they have in some places been arranged by water. Thus here and there, especially at or toward the bottom of the accumulation, distinct traces