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are therefore repeatedly introduced with others in the systematic advance of the course; and the student may gauge his progress by the increased meaning that these illustrations gain on every return.

Let us next consider the development of a deductive geographical scheme, by which external observation is to be supplemented and completed. Let it be understood at the outset that to exceed the variety of nature is an extended enterprise, a remote and ideal goal, towards which we strive. Let no excessive flight of theory carry us far from the earth and overcome us in mid-air. Let us carefully guard against an unwarranted wandering of the imagination by frequent conferences with the facts of observation, hoping to return, like old Antæus, strengthened for new efforts after every touch of Mother Earth.

The deductive geographical scheme.—It is the fundamental generalization of elementary geology to note that the lands are wasting away under the destructive attack of the weather. The hardest rocks decay; their waste creeps and washes down to lower and lower levels, never satisfied till it reaches the sea. However broad a plateau, however lofty a mountain range, it must, if time enough be allowed, be worn down to sea level under the weather; and the unceasing beat of the sea on its shores must reduce it still lower to a submarine platform. Since the remote beginning of geological time there has been time enough and plenty to spare to reduce all the lands to such a submarine platform; but as high lands still exist, it must be concluded that they are revived from time to time and from place to place by some forces antagonistic to those of subaërial denudation. In whatever way a new mass is offered to the wasting forces, let us call the forces that uplift it constructional forces; and the forms thus given, constructional forms. Let all the forces of wasting be called destructional forces; let the sea level surface, down to which a sufficiently long attack of the destructional forces will reduce any constructional form, be called the ultimate baselevel; and let the portion of geological time required for the accomplishment of this task be called a geo-