THE DRIFT-DEPOSITS OF THE NORTHWEST. 203 



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 differ- 

 ently applied by different writers, depending somewhat on the sup- 

 posed 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 char- 

 acters 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 regai'ded as geologists such as surveyors, engi- 

 neers, 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, 1 and in others em- 

 braced in what may be called the continental basin, east of the Missis- 

 sippi, has shown it to consist, in general, of the following parts, in de- 

 scending order : 



N~o. 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 propor- 

 tion 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 invari- 

 ably 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, con- 

 sists of alluvium, washed into the great valley by innumerable sti-eams 

 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 loam 

 already described. Its thickness at points remote from the river is 

 dependent on the facilities for natural drainage and wash. It may be 



1 Minnesota. 



