98 Mr, Maclure on the Geology of part of N. America. 



Art. III. — Some speculative conjectures on the probable 

 changes that may have taken place in the Geology of the 

 Continent of J^orth- America east of the Stoney Moun- 

 tains ; by William Maclure, Esq. President of Acade- 

 my of Natural Sciences, at Philadelphia, and of the 

 American Geological Society. 



Madrid, July 9, 1822. 



In the present state of our geological knowledge, there 

 are, perhaps only a few facts from which we are permitted 

 to draw conclusions respecting the former state of the earth; 

 amongst which is our entire ignorance with regard to the 

 origin or formation of the primitive class of rocks, we having 

 as yet had no opportunity of observing nature in the act 

 of aggregating or forming such rocks : the other four class- 

 es of Vulcanic, (Volcanic.'') Alluvial, Secondary, and Trans- 

 ition, we have either caught nature in the act of aggregating 

 or forming such rocks, or rocks that from direct analogy 

 are so similar in their construction, relative situation, &,c, 

 &c. as to warrant a deduction that they were most proba- 

 bly formed after this same manner. 



Water appears to be the principal agent in changing the 

 form of the earth's surface, and by the sea, lakes, and rir- 

 ers, (the most extensive mode of operation ;) when we see a 

 river running between two precipices of rocks in a deep 

 channel, whose stratification and arrangement are the same 

 on both sides of the river, we are naturally led to suppose 

 that the action of the running water wore down that chan- 

 nel, and that at some former period, the two sides of the 

 river, now separated, were contiguous and unbroken : when 

 we cast our eyes over immense tracts, such as the steppes 

 in Russia, the prairies in the United States of America, or 

 on plains that are nearly horizontal, we are tempted to con- 

 jecture that the earth took that form from the depositions 

 from water, he. &.c. he. 



The continent of North- America, east of the stoney moun- 

 tains consists of a ridge of primitive mountains, springing out 

 of the great northern primitive formations, covered to the 

 east and south-east by extensive beds of alluvial, apparently 

 the depositions of the ocean, and on the west side overlayed 

 by Transition and Secondary, filling the immense basin 

 through which the Mississippi now runs with all its attend- 

 ant streams. 



