390 DYNAMICAL GEOLOGY. 



Coast chain commencing to the south in the Coast ranges of California, and 

 continuing along the islands of British Columbia, and on the sea border 

 beyond to Mount St. Elias. 



Finally, the combination of two or more chains makes a Cordillera, as 

 the term is used in South America for the Andes. Accordingly, the Coast 

 and Sierra chains together with the chains of the Rocky Mountain summit 

 constitute the Cordillera of the Rocky Mountains. In South America the 

 term cordillera is used not only for the Andes as a whole but often also for 

 one of its long ridges or ranges or chains. Tlie combined mountain systems 

 of the whole Pacific border of ISTorth America were first called a Cordillera 

 by J. D. Whitney. 



By the above definitions, range, system, chain, are no longer interchange- 

 able terms, dependent for their use on extent or complexity of mountain 

 regions, but have fixed significations. 



8tudy of a mountain range. — Since an individual mountain range has great magni- 

 tude, and commonly great complexity through its long series of involved flexures and 

 faults, and through the excavating work of running waters, investigation requires a long 

 and searching study of the structure as a whole, — that is, as an individual. The geological 

 examination of a single ridge of a range may afford conclusions as to the fact of upturnings, 

 flexures and faults and may obtain evidence as to the force concerned, and perhaps settle 

 the question of the foliation, or bedding, of the schists of the ridge, if any are present. But 

 it can afford no general conclusions as to the range ; and a petrological investigation would 

 accomplish still less. A single section across the range would afford facts, but no general 

 results ; for the flexures may vary every few miles, new faults appear and other rocks 

 come out to view. The student should make his sections not merely in one, or a dozen 

 transverse lines, but in as many lines as possible in all directions, studying positions of 

 strata, and noting the changes they undergo from ridge to ridge until the connection of 

 each ridge with every other in the general system of warping has been ascertained. 

 Further, this study should be carried on until the true limits of the mountain individual 

 as far as possible are ascertained. And if the range is more or less metamorphic, the belt 

 of maximum metamorphic change should be studied out, and the fringe of diminishing 

 change, on one or both sides. A ridge of upturned rocks, whether Archaean or of later 

 date, is almost invariably evidence of the existence, in the region, of a mountain range 100 

 to 1000 miles long, or more ; and this should be assumed to be a fact until the contrary is 

 proved. 



With the completion of the investigation there will be little further reason for ques- 

 tionings about the fact of pressure and movements as a source of dynamical effects ; and 

 if the beds are metamorphic, none as to the source of the heat that produced the meta- 

 morphism. But it remains for petrology to complete the work by investigating the special 

 characteristics of the metamorphic changes, their relations to the positions of the beds, 

 the minerals due to the original metamorphisra and the results of later changes, besides 

 other points in the history, for light upon which geology is dependent on its kindred 

 science. 



Further, a mountain range being a very large individual, — a length of a thousand 

 miles and breadth of more than a hundred being common, — three such individuals 

 cannot exist on a single area of 50 miles square. When, therefore, indications of three 

 or more periods of upturned rocks are announced, as indicated, by unconformabilities 

 in any limited region of upturned crystalline or uncrystalline rocks, Archsean, or 

 others, it is quite certain that the unconformabilities are in part only unconformities 

 through faults, or overlaps, or erosion, which have little epochal significance. 



