Page:The Journal of geology (IA journalofgeology11893univ).pdf/436

(d) The Tennessee type, prevailing in Kentucky, Tennessee, Arkansas, Mississippi and Alabama, has the highest rainfall the last of winter, while the minimum is in mid-autumn. (e) The Atlantic type, covering all the coast save New England, is one where the distribution throughout the year is nearly uniform, with a maximum precipitation after the summer solstice, and a minimum during mid-autumn. (f) The St. Lawrence type is characterized by scarcity during the spring months, heavy rainfall during the late summer and late autumn months, with a maximum during November.

The regions lying between these several type-regions have composite rainfall types, resulting from the influence of two or more simple types.

The drainage of the Eastern Mississippi basin in post-carboniferous was in all probability consequent upon the tilting which accompanied the stronger folds of the Appalachian revolution in the east. The present drainage is found to accord in the main with this hypothetical post-carboniferous drainage, but several streams depart quite widely from it.

(a) The great drainage lines of the St. Lawrence basin are structural valleys developed along the strike of the softer Paleozoic strata, and at right angles to the original surface. The streams seem, therefore, to have adjusted themselves to the differences in hardness and structure of the beds discovered. (b) The Ohio and Cumberland rivers cut directly across the Tennessee and Cincinnati anticlines. The most probable explanation is that the rivers were superimposed upon the arched and eroded Silurian rocks from a thin cover of carboniferous beds—now entirely removed. (c) The Upper Mississippi does not follow the dip of the rocks to the southwest, but follows the strike to the southeast. This part of the river probably dates from the elevation of the plains on the west and the Appalachians on the east, which marked the close of the Cretaceous and which left a broad north and south valley. (d) The author finds good reason to believe that the Lower Mississippi, in post-carboniferous times, flowed west through Missouri and Arkansas. The present course was probably taken at the close of the Cretaceous in consequence of elevations on the west and east, and possible depression in the south.

The Cretaceous base-level recognized by Davis on the Atlantic slope can be traced more or less discontinuously, and remnants of it are believed to exist in Kentucky, Tennessee, Wisconsin, Minnesota and Arkansas. But in general the work of the Tertiary cycle has obliterated almost all evidence of it on all but the hard sandstones and conglomerates of the Paleozoic series.