Page:The Journal of geology (IA journalofgeology11893univers).pdf/431

theoretical evolution applicable only to the land of man's origin. The present stage of civilization is certainly not an immediate derivative of the next preceding, but has been imposed upon it unconformably, so to speak, and disjunctively. It is intrusive or superposed, and not derivative. So it is probable that the peculiar phases of the higher civilizations found in Central and South America were intrusive and not derivative. It is, therefore, not improbable that the entire succession of civilizations on the American continent consists of a series of intrusions or superpositions from the west and from the east, overlapping each other unconformably and disjunctively. They can, therefore, be worked out safely upon no theory of genetic succession. Each factor must be determined by means of its own inherent evidence.

Professor James D. Dana has a short article in the November number of the American Journal of Science^[1] touching upon the recent discussion of the divisibility of the glacial period, in which he draws forth generalizations on two important lines, viz., (1) the personal attitude of writers on the subject, and (2) the difference between the glacial phenomena of New England and of the upper Mississippi basin. These seem to us to lie in the right direction, in the main, but in both cases to have somewhat missed the truest lines of distinction and to have fallen short of the most significant features. Professor Dana draws attention to the divergent views of New England and of western glacialists, and concludes that there must be some difference in the phenomena of the two regions to account for the differences of view. This seems to us very true and very important. The difference in the phenomena is, however, we think much more radical, and, at the same time, much more simple than that suggested by Professor Dana. It is, to our view, simply this: In New England only the latest epoch of the glacial period is distinctly repre-