162 
GEOLOGY OF THE FIRST DISTRICT, 
miles west of Commac, the same characters of the drift'hills are observed, but on a smaller 
scale. 
It is only where the drift deposits have a considerable thickness, that the hilly character of 
the drift is observed. Where it is thin, it does not give any marked character to the country, 
but serves to fill up the irregularities that would otherwise exist upon the rocky surface, and 
give a smoother outline. A large portion of the surface of the earth in the First district of 
New-York, and in New-England, Ohio, and all the western and northwestern States and Ter¬ 
ritories, is covered by a mantle of the drift deposits. The drift is frequently covered, in the 
valleys and depressions of the surface, by the quaternary and alluvial deposits; but it may 
perhaps be safely said, that three-fourths of the surface shows this deposit, often very thin, 
but frequently of considerable thickness. 
In the prairie regions of the western and northwestern States and Territories, this forma¬ 
tion has a great development, covering almost the whole face of the country, and showing 
strongly marked topographical features. Where the drift alone forms the surface, and has 
some considerable depth, it forms undulating plains, dotted with numerous boulders and blocks 
upon its surface, and in some localities showing the same characters of hills and valleys, as 
have been described on Long island and other localities. This is the case in some parts of 
the plains at the base of the Coteau de Prairie.* 
These vast plains, composed of drift, covered in some places by alluvial and quaternary 
deposits, are boundless to the eye, and the traveller passes across them guided only by his 
compass, like the mariner upon the ocean, and the same mirage is there observed. When a 
distant island of woodland first makes its appearance in the horizon, showing the tops only of 
the trees, a person unacquainted with such. deceptions would be apt to think he saw a broad 
lake intervening, and perhaps describe such in his travels. Objects are strangely magnified, 
and a small swell at a few hundred yards distance, with some weeds upon its summit, might 
easily be mistaken for a high range of land covered with trees.! These appearances were 
more marked and distinct where the prairies had been recently burnt over, and the blackened 
surface, heated by the sun’s rays, caused more than ordinary refraction in the atmosphere. 
The light and shade visible on an unblackened surface, and almost all objects that might have 
been used as sources of comparison, were wanting. 
The terraces that bound these vast plains of one level, are but the edge of others as vast, 
and in the horizon have the aspect of a distant coast; and this appearance has given the name 
to the Coteau de Prairie. These terraces in some places present another appearance like a 
coast line, as perhaps they were in some former time, probably at the quaternary period. 
Boulders in immense numbers are seen on the faces of some of these terraces, lying in a 
* Vide Featherstonhaugh’s Geological Report, 1836, p. 153. “Sand hills” should have been described as hills of gravel, 
pebbles, boulders and blocks. M. Vide also Mather’s Geological Reports to the Secretary of War, MSS. 
t Such facts, and numerous others, were observed by Mr. Featherstonhaugh and myself in a geological reconnoisance to the 
Coteau de Prairie in 1835, and are described in our reports to the Secretary of War. Vide Mr. Featherstonhaugh’s Geological 
Report, 1836, pp. 153, 156; and Mather’s MSS. Geological Report to the Secretary of War. 
