ORIGIN OF RANGES 335 



in descending order of magnitude, the smallest of them being 

 visible only with the microscope. (3) Dynamic metamorphism 

 is an almost universal feature of mountain ranges, the transforma- 

 tion of the rocks being in proportion to the intensity of the plica- 

 tion. The microscope gives eloquent testimony to the enormous 

 forces which have been at work, by showing how the minerals have 

 been mashed and flattened, rendered plastic and flowing like wax 

 in a hydraulic press. (4) Masses of igneous rocks are very often, 

 though not always, associated with mountain ranges, and many 

 such ranges have a core of igneous rock, often granite, with 

 strata flanking it on both sides. 



Origin of Mountain Ranges 



The manner in which mountain ranges have been formed must 

 be deduced from a careful study of their structure, for no one 

 has ever witnessed the process of that formation. Mountain 

 building may be going on at the present time ; indeed, there 

 is no reason to suppose that it is not, but so slowly is the work 

 carried on that it withdraws itself entirely from observation. 

 Nevertheless the general course of events may be inferred with 

 much confidence from the structure of the range. 



The first step in the formation of a mountain range must evi- 

 dently be the accumulation of an immensely thick body of strata. 

 This, of course, must have taken place under water, and the only 

 body of water large enough is the sea. Furthermore, our studies 

 of modern marine deposits have taught us that thick strata can be 

 accumulated only in rather shallow water and parallel with shore- 

 lines. This shoal water origin of their strata is confirmed by the 

 examination of actual mountain ranges, where we find great masses 

 of conglomerates, ripple-marked and sun- cracked sandstones and 

 shales, and abundant other testimony of deposition in shallow 

 water. To accumulate thick strata in shoal water the bottom 

 must subside as the sediments are piled upon it, else the water 

 would be filled up and deposition cease. Such a sinking trough 

 is a geosyncline, and in geosynclines filled with sediments is the 



