90 Transactions of the American Institute. 



deserves, perhaps, in comparison with the whole ; but we. know so 

 well the strata of which it is composed, and these have been studied 

 more thoroughly, in all their phases, than any other part of the con- 

 tinent. It is the more easy, therefore, to draw our comparisons and 

 illustrations from this than from any other of the great areas of which 

 the entire continent is composed. And, moreover, the illustrations 

 offered in this continental period are, as I conceive, applicable to all 

 other parts of the continent, and to other continents ; the conditions 

 in all having been similar, and the laws operating are everywhere the 

 same. 



In this discussion, however, we may consider, for a few moments, 

 the condition of the strata composing the mountain ranges. The 

 beds or strata are tilted, folded or contorted by some action taking 

 place subsequent to their deposition, and after they had become par- 

 tially or entirely solidified. They are likewise more or less crystal- 

 line in texture. This crystalline texture, we say, is due to metamor- 

 phism, a change produced upon sedimentary strata by a moderate 

 degree of heat ; and we conceive that the necessary augmentation of 

 temperature may have been acquired in the depression of these beds 

 to the depth of five or six miles below the sea level, which depth 

 must have been reached by the lower beds even during the accumu- 

 lation of the higher strata. However, we know that this change is 

 not due entirely to the influence of extraneous heat, but to the nature 

 and composition of the beds thenriselves ; for we sometimes find the 

 lower beds entirely unchanged, while succeeding beds, several hun- 

 dred feet higher in the series, are highly metamorphosed. AVithout 

 discussing this point further, we may remark that metamorphic sedi- 

 mentary strata usually occur only where the accumulations are very 

 considerable, and that condition is most complete where the strata 

 have the greatest thickness. 



In considering the geological structure of this part of the country, I 

 present before you a section of the strata composing the Apalachian 

 chain as shown by Prof. Rogers in his Geological Survey of Pennsylva- 

 nia. This section, and others of similar character made by myself 

 across eastern New York, and parts of Massachusetts and Yermont, 

 exhibit the strata precisely as they exist, and not as represented by 

 any theoretical views whatever. Such sections not only show the 

 present position and relations of the rocky strata, but they demon- 

 strate most conclusively that there is no line of outbreak from 

 internal forces, nor the intrusion of an}' rock whatever to produce 



