TYPES OF MOUNTAIN CHAINS 91 



terized by the great diversity of igneous rocks, contrasted with the elon- 

 gate Andean type, which is distinguished by the monotony of the effusive 

 rocks over large areas. 



Mountain chains of two types. — The contractional hypothesis is doubtless 

 the best we have for the explanation of mountains ; but in this time of 

 the foundering of the whole crust of the earth it was greatly overworked, 

 and it would be a distinct relief if a large and peculiar group of orographic 

 forms could be thus explained in another way, and one which involves 

 perhaps less depth of movement and can be best harmonized with the 

 great diversity of the volcanic products. We would put in the group of 

 mountains of flowage those of the zone we are studying and the Asian 

 side of the Pacific band. They are convex in direction from the center 

 of motion, and have often flowed toward the sea. They are most folded 

 on the convex side, farthest from the center of motion, and lessened fold- 

 ing, stretching, and Assuring with frequent outpour of lava appear on 

 the concave side nearest the center of motion. 



The other group, including the Appalachians, the Urals, and the Cor- 

 dillera, are formed by tangential thrust of the sinking sea-bottom, are 

 straight or even concave (Urals, and Appalachians in crossing the Missis- 

 sippi) on the side away from the center of motion, and have always 

 moved from the sea. They are most folded on their straightest or convex 

 side, nearest the center of motion where the force was applied, and do 

 not have volcanoes on the concave side, and the folds die out on the side 

 farthest from the center of motion, as in the Cincinnati uplift, or Parma, 

 as Suess calls these low folds from the corresponding one in the Urals. 

 The Great Basin topography we may hold with Suess * and Lundgren to 

 be caused by the collapse of a great Parma extending from the Wasatch 

 to a line east of the Rocky mountains, and the Uinta and the smaller 

 peaks east to the Rockies to be horsts remaining at the higher level, the 

 whole formed as the ice in a drained pond collapses around the large 

 rocks on the bottom. 



The Pacific zone — We have finally to consider the chains bounding the 

 north Pacific ocean (plate 14, a and 6, page 78). Too great stress has 

 been laid on the distinction between an Atlantic and Pacific coast type, 

 and the violence of the forces around the Pacific have produced a contin- 

 uous line of volcanoes, which disguises the diversity between its two shores. 

 Indeed, in the article given below Suess, the author of the distinction,seems 

 to recede from the position that the mountain chains have flowed toward 

 the sea on both sides of this ocean. There seems to be substantial agree- 

 ment among the American and Canadian geologists who have studied on 



* Das Autliz der Erde, vol. i, p. 734. 



