INTRODUCTION. 
T'sE country which, during the conduct of this Survey, has been more or less 
carefully examined, and of which the geological features have been determined, and 
are, on the General Map, exhibited by colouring separately each formation, is the 
most extensive ever reported by any geologist or geological corps in this country; 
including as it does more than four times as much territory as the State of New 
York, and being about twice and a half as large as the Island of Great Britain. 
Wisconsin, except its eastern portion on Lake Michigan, Minnesota, and Iowa, 
were embraced in my instructions. The Map, it will be seen, extends somewhat 
beyond these bounds, including a portion of Northern Illinois, and also of Northern 
Missouri. These additions were necessary to a proper understanding of the 
formations of the district expressly required to be explored ; and they place before 
the eye, at once, as well the size and shape of the Iowa and Missouri coal-field, as 
its relation to that larger coal-basin, heretofore (to wit, in my Report of 1839) laid 
down by me as the Illinois coal-field. 
With these additions, the Map reaches from latitude 38° to latitude 49°; and 
from longitude 89° 30’ to longitude 96° 30’. In other words, it has a length from 
north to south of upwards of seven hundred and fifty miles: from St. Louis to the 
British line; and an extreme breadth of about three hundred and fifty miles: 
embracing the Mississippi and all its tributaries, from its source to its junction with 
the Missouri; the Missouri, as high as Council Bluffs; the Red River of the North, 
from its source to the northern boundary of the United States; together with the 
northern and southern shores of Lake Superior, from Fond du Lac, north to the 
British dominions, and east to the Michigan line.* 
The average width of the territory thus laid down being about two hundred 
and seventy miles, its area exceeds two hundred thousand square miles. 
* The recently set off reserve, on the Mississippi, south of Crow Wing, and now ceded to the Winne- 
bagoes, must be here excepted. Covered to a great extent with drift, without promise for the geologist, 
and likely to remain Indian property, its examination would have been little valuable to science, and 
useless to the Department. 
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