VALLEY OP THE MISSISSIPPI. 19 



the ambiguity inherent in all conglomerates, which 

 merely mark the transition of one formation into that 

 of another, and are thus almost intermediate betwixt 

 every species of transition whether general or partial. 

 There is, I think, reason to believe, that most of the 

 finer chalcedonic geodes, which appear in the form 

 of pebbles of various sizes, originate almost uniformly 

 in those transition rocks which we term amigdaloids 

 and conglomerates, and though porphyries, as ap- 

 proaching more nearly to the class of rocks called 

 primitive, are artificially distinguished from them, 

 there exists, in fact, no such natural precision of 

 limit.* At all events, the presence of these chal- 

 cedonic debris, if not more remotely adventitious, 

 would appear to point out in this quarter, the termi- 

 nation of the calcareous platform, somewhere below 

 the sources of the Mississippi as well as those of the 

 Missouri. 



Descending the St. Lawrence, or rather its chain 

 of lakes, we perceive even along the southern coast 

 of the Huron, very intelligible indications of the ap- 

 proaching termination of this secondary formation, 

 in the vast beds, as I may call them, of adventitious 

 granitic rocks, which for more than one hundred 

 miles in succession, continue to line its shores. 

 Many of these blocks, which are in places collected 

 and extended into the lake for ten or twelve miles 

 together, are of a magnitude so enormous, as to have 



* One or two specimens of hyaline calcedony, I once found 

 on the gravel bars of the Missouri, imbedded in a white Jasncv 



