HYPOGEIC WORK. 



353 



sometimes large displacements in the same upthrust way. The direction of 

 dip of the plane of fracture, as the figures show, is, in the case of a synclinal 



325. 



326. 



327. 



bend, the reverse of that in the anticlinal. 



In subjecting to vertical pressure a square block of 



wax, having a breadth of five and a half inches and a 



height of about a foot, an oblique diagonal fracture was made with some 



bulging of the sides ; and, adjoining the 

 fracture, as a consequence of the molecular 

 movements in the bulging, fine rectangular 

 cracks were produced, like a delicate net- 

 work. 



Cubes of hard rock under vertical press- 

 ure usually break oif at the angles and 

 edges, leaving two rounded cones with their 



apexes at the middle ; but a tabular block of limestone was reduced by Daubr^e 



to vertical prisms and plates. 



Characteristics of Some Typical Mountain Ranges, 

 1. The Appalachian Mountain Range. 



The structure of the Appalachian Mountains was first investigated by 

 Professors W. B. and H. D. Rogers in connection with geological surveys 

 of the States of Virginia and Pennsylvania; and their results (1842) 

 gave many fundamental principles to orographic science. 



Fig. 328, A, B, sections of part of the belt in Virginia, by H. D. Campbell, 

 afford a general idea of the system of flexures (1893). Each represents the 

 rocks for a breadth of about 10 miles across the range, in Rockbridge and 

 Bath counties. Between the two sections there is an interval of about eight 

 miles. The numbering of the formations corresponds with that on page 410 ; 

 the limestone areas are blocked, the shales ruled, and the sandstones dotted. 

 Farther to the southeast, in the same line, there are closely crowded over- 

 thrust flexures. 



In the construction of the mountain range from New York to Alabama 

 (1) the whole Paleozoic series of strata to the floor of crystalline Archaean 

 rocks — in some parts 40,000 feet thick — were involved in the system of 

 flexures ; (2) the flexures are generally parallel to the axis of the mountain 



DANA'S MANUAL — ^23 



