TYPES OF OROGRAPHIC STRUCTUKE. 9 



CHARACTERISTICS OF THE OROGRAPHIC STRUCTURE OF THE 



THREE PROVINCES. 



TYPES OF GEOGRAPHIC STRUCTURE. 



It seems convenient to give a general account of the types of oro- 

 graphic structure in the region under consideration before characterizing 

 each province by its special type. 



In this discussion I wish to use certain terms with a restricted or rela- 

 tive meaning ; i. e., in treating of anticlinal and synclinal flexures I shall 

 speak of those portions of the sedimentary beds which are adjacent to the 

 anticlinal axes as having been upheaved, and those portions near their 

 synclinal axes as having subsided. Again, "in blocks which are bounded by 

 faults and tilted, I shall speak of such portions as are at a higher level as 

 having been uplifted, and portions occupying a lower level as thrown. In 

 such cases I do not wish to commit myself to any theory of upheaval or 

 collapse in the change of the relation of the several parts of these beds to 

 the center of the earth. 



In treating of the structure of the mountains under consideration it is 

 necessary to distinguish two great classes, viz, those composed of sedimen- 

 tary strata, 'altered or unaltered, and those composed of extravasated 

 material. 



MOUNTAINS COMPOSED OF SEDIMENTARY STRATA. 



I. APPALACHIAN STRUCTURE. 



The structure of the Appalachian Mountains, with closely appressed 

 folds and axial planes tipped back from the sea, the modifications of these 

 folds by faults, and the primary and concomjtant forms of tile mountains, 

 have been clearly explained by the Messrs. Rogers and later writers, and 

 have formed the basis of many discussions concerning geological dynamics. 

 This Appalachian structure needs no further mention here, as it is a type of 

 structure which so far has not been found in the region described above, and 

 should it be found hereafter it will simply be an exceptional type to those 

 known to prevail. 



