HYPOGEIC WORK. 351 



Thus tlie earth is still undergoing changes from paroxysmal movements 

 and prolonged oscillations. The changes, while probably more restricted 

 than in the ages of progress, are yet the same in kind. 



Chakacteeistics of Disturbed Regions and Mountains. 



General Explanations. 



1. Tlie general range of effects. — A disturbance, in geological usage, is an 

 event in which rocks — formations of wide extent — are moved, and more or 

 less fractured in the process. Over some great areas they have been shoved 

 up or depressed with little variation from horizontality ; and over others 

 there have been profound flexures and faults involving thousands of feet 

 of strata throughout regions hundreds of miles wide and thousands long. 

 Explanations and illustrations have already been given of uptiirned beds, 

 flexures, faults, and flexure-faults (page 99), and of the metamorphism 

 and vein-making, which have attended great mountain-making movements. 

 The object of the present chapter is to present all the various orographic 

 phenomena in their relations as they occur combined in the structure 

 of mountain ranges and systems of ranges, and to explain, so far as is at 

 present possible, the origin of orographic movements and of the resulting 

 structures. 



In the first place, some facts in molecular physics of fundamental impor- 

 tance as regards flexures, fractures, and faultings of solids, are here briefly 

 illustrated, and then follow descriptive examples of several mountain-struct- 

 ures, as a prelude to the discussion of the question of origin. 



2. The flow of solids. — Solid metal and rock, when under pressure, as 

 first illustrated by Tresca, yield through molecular movement, and may thus 

 become compressed, drawn out, flexed, and otherwise deformed. The yield- 

 ing is very much like that in a bent bag of shot, through movements in 

 the shot. In the case of metals, ice, and rocks of even texture, the change, 

 if slow enough, may take place without fractures. In the bending of a 

 mass of rock or ice by gravity, molecules of one side push the adjoining, 

 and these others throughout the mass, until the adjustment is complete. 

 Hence the density is nowhere changed. The flow of metals is now util- 

 ized extensively in the shaping and punching of cold metal for various 

 purposes in the arts. In experiments at the Stevens Institute, Hoboken, 

 by Mr. David Townsend (Journal of the Franklin Institute for March, 

 1878), rectangular blocks of iron, accurately planed and measured (being 

 made about 1-75 inches wide and thick, and 2-5 inches long), were punched 

 cold through the center with a punch yV ^^ ^^ inch in diameter. The 

 core which came out (Fig. 323) was only ii- of an inch (instead of 1-75 

 inches = -f-l) long ; and yet, like the punched block, it was essentially un- 

 changed in density. Mr. Townsend's experiments and measurements show 

 that six tenths of the metal which had filled the hole had moved off lat- 



