Page:Popular Science Monthly Volume 68.djvu/322

318 review of his opinions may be given in some detail. Beginning with the later Tertiary times, he believed the following sequence of events to have been established.

(a) During Miocene and Pliocene epochs a continent several hundred feet lower than now, the ocean reaching to Louisville and Iowa, with a subtropical climate prevailing over the lake region, the climate of Greenland and Alaska being as mild as that of southern Ohio at present.

(b) A preglacial epoch of gradual continental elevation, which culminated in the glacial epoch, when the climate of Ohio was similar to that of Greenland at present, and glaciers covered a large part of the surface down to the parallel of forty degrees.

(c) This period followed by another interval of continental subsidence characterized by a warmer climate and melting glaciers and by inland fresh-water seas filling the lake basins, in which were deposited the Erie and Champlain clays, sands and boulders.

(d) Another epoch of elevation which is still in progress.

The sheet of clay and boulders which was found directly overlying the polished surface of the rocks over so large a part of the state, now known under the name of till and boulder clay, he described under the general name of glacial drift, while the loose boulders which he found scattered indiscriminately over the surface, frequently resting on the fine stratified clays, were known under the name of iceberg drift.

If, he wrote, we restore in imagination this inland sea, which we have proved once filled the basin of the lakes, gradually displacing the retreating glaciers, we are inevitably led to a time in the history of this region when the southern shore of this sea was formed by the highlands of Ohio, etc., the northern shore a wall of ice resting on the hills of crystalline and trappean rocks about Lake Superior and Lake Huron.

From this ice wall masses must from time to time have been detached, just as they are now detached from the Humboldt glacier, and floated off southward with the current, bearing in their grasp sand, gravel and boulders—whatever composed the beach from which they sailed. Five hundred miles south they grounded upon the southern shore—the highlands of now western New York, Pennsylvania and Ohio, or the shallows of the prairie region of Indiana, Illinois and Iowa. There melting away and depositing their entire loads.

The loess, as one would naturally expect from the foregoing, was looked upon as the finer sediment deposited in the quiet waters of one of these inland seas, to which the icebergs had no access. The lake basins, with the exception of that of Lake Superior, were regarded as excavated by glacial action—thus agreeing with Logan.

The views of Orton, who succeeded Newberry as state geologist, were not widely different, and, briefly expressed, were as follows:

Threefold divisions of glacial time may be considered as