perplexing phenomena concerning the larger features of earth than any one of the great themes mentioned and perhaps more than all of them combined. It projects the imagination backward to the beginnings of geologic history; and it carries it forward to the end of time. In the lineaments of our dead moon it may be we behold the final effect of eolic powers.
Although perhaps not wholly the unaided work of any one man or group of men, the generalization of regional eolation is first distinctively American in origin. As such it seems not too much to say that it is allotted to stand as one of the far-reaching achievements of our century. It is doubtless the last of the great discoveries in geologic science to be attained by purely observational methods. The future advancements in earth-study must be quantitative instead of qualitative in character. They must be the direct outcome of mathematical investigation, of the rigid application of the new physico-chemical laws, and of the complete evolution which the discovery of radio-activity has imposed.
This, then, is briefly a statement of the theory of regional eolation.