ORIGIN OF MOUNTAIN RANGES. 5 5 I 



and a spreading there from a centre, of a series of elastic earth- 

 waves. This is the Formal theory. It explains the immediate facts 

 observed here on the surface, but no more. But still remains the 

 question, What is the cause, deep down below, of the concussion 

 which determined the series of earth-waves. This, the physical 

 theory, is far more doubtful. Or the theory may be made still 

 deeper and proportionately more doubtful. If our theory of the 

 cause of the interior concussion be the formation of a fissure or 

 readjustment of a fault, as seems in many cases probable, there 

 would still remain the question of the cause of great fissures and 

 of their subsequent readjustment by slipping. This is probably 

 as far as geological theory would go: for although cosmogony 

 may go still farther, the interior heat of the earth is usually the 

 final term of strictly geological theories. 



I have made this long detour because I wish to keep clear in 

 the mind these two stages of theorizing in the case of Mountain 

 Origin. The formal theory is already well advanced toward a 

 satisfactory condition ; the physical theory is still in a very 

 chaotic state. But these two kinds of theories have been often 

 confounded with one another in the popular and even in the 

 scientific mind and the chaotic state of the latter has been car- 

 ried over and credited to the former also ; so that many seem to 

 think that the whole subject of mountain-origin is yet wholly in 

 air and without any solid the foundation. 



I. FORMAL THEORY. 



A true formal theory, keeping close to the immediate facts 

 in hand, must pass gradually from necessary inferences from 

 smaller groups, to a wider theory which shall explain them all. 



Inferences from i and 2, i. e., Thickness and Coarseness of Sedi- 

 ments. — The thickness of mountain sediments, as we have seen, is 

 greatest along the axis and grows less as we pass away from that 

 line. Now where do we find lines of very thick sediments form- 

 ing at the present time? The answer is: On sea bottoms 

 closely bordering continents. The whole washings of continents 

 accumulate very abundantly along shore lines and thin out sea- 



