When stratified and assorted, they have sometimes been denominated modified drift; when not assorted, unmodified drift. But these terms require considerable caution in their use, since they have been differently applied by different writers, depending somewhat on the supposed cause of the assortment witnessed in modified drift, and since the assorted and non-assorted portions of the drift are not uniform, either in their positions in the great mass of the deposit, or in the characters they generally possess.
The character and nature of the drift in the Northwest are very largely misapprehended. This is true, not only among those who might not strictly be regarded as geologists—such as surveyors, engineers, lecturers, and public literati—but even among those who have given considerable attention to the study of fossils and rocks. These misapprehensions, so generally spread among the people, are largely due to the industry of the authors of certain theories concerning its origin, in spreading their views before the public. A plausible theory, moreover, has a great influence in its own favor.
A pretty careful study of the drift in this State, and in others embraced in what may be called the continental basin, east of the Mississippi, has shown it to consist, in general, of the following parts, in descending order:
No. 1. Surface Soil.—This, of course, presents all the varieties due to local influences. Over large portions of the Northwest it is a fertile black loam, highly arenaceous, and supplied with a considerable proportion of carbon in a state of minute subdivision. This arenaceous loam passes into a more gravelly loam on the brows of knolls and in rolling land. It is also sometimes replaced by a gravelly clay. This is the case in large portions of the State of Michigan, and in Central and Southern Ohio. This is the fact in Northern Indiana and in Central Minnesota. The gravel prevails in wooded and rolling districts. In treeless districts the sandy element is more common, making a black loam. In valleys and along streams the soil is alluvial. It is invariably fine, nearly free from stones and bowlders, and very fertile. It is what is popularly known as "made land," and comprises those parts of the drift of the highlands that are susceptible of transportation by running water. That which is known as the "bluff-formation," lining the Mississippi, both in Minnesota and in the States farther south, consists of alluvium, washed into the great valley by innumerable streams from the adjoining country, at a time when the volume of the river was immensely greater than now. The same materials are now spread over the farms of Southern Minnesota, over much of Iowa and Illinois, over Northern Missouri and all the Far West, to the Rocky Mountains. It lies there also in the form of fine sand, and constitutes the loamalready described. Its thickness at points remote from the river is dependent on the facilities for natural drainage and wash. It may be