Hi'POGEIC WORK. 391 



In addition, it should be remembered that the unconformabilities between the upturned 

 rocks of a mountain and tliose underlying are usually confined to the mountain region. 

 A score or so of miles to one side, the rocks may often be found resting beneath the same 

 strata, perhaps horizontally, with perfect conformability between them. The unconform- 

 abilities are on this account none the less important as time-boundaries in geological 

 history. 



When, in consecutive epochs of mountain- making, the upturned strata of the later 

 epoch have been thrust up against those of the earlier, by force acting in the two cases from 

 the same direction, the two sets of strata will have more or less nearly the same strike. 

 But their unconformability may possibly still be proved (1) by difference in dip ; (2) by 

 difference in kinds of rocks, when the rocks are studied over a long belt in the line of 

 strike ; and (3) by fossils, if the beds are fossiliferous. But when the strata are metamor- 

 phic, and fossils are therefore absent, the difficulties are great. Examples occur in western 

 Connecticut and eastern New York, where the metamorphic Taconic rocks come into con- 

 tact with Archaean. The first and second of the above criteria may still be available, 

 though with great uncertainty ; the second may be used especially when the two sets of 

 strata differ in grade of crystallization or metamorphism, or in the presence of some dis- 

 tinctive mineral masses, as of metamorphic beds of iron ore. The belt should, further, be 

 traced along the range of outcrops in order to find, if possible, a region where there is a 

 bend in the strike ; for at such a bend the two sets of strata probably would not be found 

 to bend alike ; and to make the investigation complete, all possible strikes and dips should 

 be measured and plotted on a large map of the region. Special care is needed in order that 

 "unconformity produced by a fault is not mistaken for true unconformability or that in 

 the bedding. 



3. GENERAL RESULTS OF OROGRAPHIC WORK. 



1. Effect of orographic work on the eartKs circumference. — Faults and 

 plications are a measure of the shortening of the earth'' s circumference that has 

 taken place in an orographic crisis. During the ages of preparation, the 

 amount of shortening in the making of the geosyucline has been small ; for 

 the slowly accumulating strain reduces widths only by the difference between 

 the shallow arc and its chord. But at the collapse, as already shown, the 

 amount has been a score or more of miles : 74 for the Alps (Heim) ; 44 for 

 the Appalachians in Pennsylvania ; 25 for the Laramide Kange in British 

 America (McConnell). 



The line of the Appalachian Eange is transverse to a zone of the globe 

 having a IST.W.-S.E. direction ; and the Taconic Range and the Acadian of 

 Nova Scotia and New Brunswick widen this zone northward. The short- 

 ening of the earth's circumference for all these ranges was not east-and-west, 

 but in the direction of this zone. In this zone the Archaean nucleus is to the 

 northwest; but to the southeast lies the Atlantic, in its long range between 

 North and South America. In western America, where the mountains made 

 range northwestward instead of northeastward, the shortening was in the 

 direction of a zone N.E.-S.W. in course. It was the same zone of the globe 

 that includes the Alps. The whole amount of shortening on the Atlantic 

 border was probably not over 50 miles along the course of the zone; and 

 on the Pacific border for the Laramide and other systems later than the 

 Archaean, not over 75 miles. 



