GLACIO-AQUEOUS ACTION IN NORTH AMERICA. 205 



slightly inclined layers. Such seems to have been the origin of 

 those deposits of stratified clay and sand, that occur in almost 

 every basin of much size in the northern parts of this country. 

 The clay is blue, and the iron in it, which amounts to nearly ten 

 per centum, is usually in the state of protoxide. Its beds vary in 

 thickness from a very few feet up to one hundred ; and the laminae 

 are rarely more than half an inch thick. The sand almost inva- 

 riably lies above the clay, and is rarely more than fifteen to twenty 

 feet thick. At the upper part of the clay bed, we find occasionally 

 alternations of layers of fine sand ; but usually the transition from 

 clay to sand is quite rapid, and accomplished in a vertical direction 

 of a few feet, or more frequently of a few inches. 



It is rare to find alluvial deposits overlying the sand above 

 described as belonging to the drift period : for the alluvium seems 

 generally the result of the wearing away of the sand and clay, 

 though we frequently find clay beneath what we know to be an 

 alluvial deposit. I am not aware that our rivers at present deposit 

 proper clay ; certainly not that blue clay which underlies the sand. 

 Neither is the alluvial deposit commonly of coarse sand, but rather 

 of fine sand ; forming a sort of loam, often containing consider- 

 able organic matter. Indeed, our alluvial deposits contain organic 

 remains, both vegetable and animal : but those connected with 

 drift, very rarely contain them. And I hardly know of a better 

 distinction between the sand and clay of drift and alluvial de- 

 posits, than to draw the line where we find the organic relics 

 begin. For these seem to mark a change in the state of the sur- 

 face, and the waters, favorable to organic existence. It is evident, 

 however, that as the barriers of detritus gradually wore away, the 

 lakes and ponds would be reduced in size ; but depositions would 

 continually go on, until the present state of the surface was at- 

 tained. Perhaps it would be better to consider the alluvial period 

 as commencing with the deposition of the lowest beds of clay. 

 We should then have a clear hthological distinction between drift 

 and alluvium. But on the other hand, the clay and sand were 

 evidently deposited previous to the last creation of animals and 

 plants on the globe ; since none of them are found in this forma- 

 tion ; and, therefore, the period of its deposition seems more 



