ORIGIN OF MOUNTAIN RANGES. 549 



very conspicuous orographic features. I have sometimes doubted 

 whether they should be called ranges at all ; but when we reflect 

 that at least 10,000 feet of the height of the Sierra is due to 

 normal faulting, it seems impossible to withhold the term. Thus 

 mountains may be divided into two types, viz., mountains formed 

 by folding of strata and mountains formed by tilting of crust- 

 blocks. The structure of the one is atiticl'mal or ^z'clinal, of the 

 other mojiocMmX. The Sierra probably belongs to both types. 

 It was formed at the end of the Jurassic as a mountain of the 

 first type, but the whole Sierra block was tilted up on its eastern 

 side without folding, at the end of the Tertiary, and it then 

 became also a mountain of the second type. 



A complete theory must explain this type also ; but since 

 from its exceptional character it must be regarded as of subor- 

 dinate importance, we shall be compelled to confine our discussion 

 to mountains of the usual type. 



EXPLANATION OF THE PRECEDING PHENOMENA. 



In all cases of complex phenomena there have been many 

 theories, becoming successively more and more comprehensive. 

 The citadel of truth is not usually taken at once by storm, but 

 only by very gradual approaches. First comes the collection of 

 carefully observed facts. But bare facts are not science. They 

 are only the raw material of science. Next comes the grouping 

 of these facts by laws more or less general. This is the beginning 

 of true science. Every such grouping or reducing to law is a 

 scientific explanation, and therefore in some sense a theory. At 

 first the grouping includes only a few facts. The explanation or 

 theory lies so close to the facts as to be scarcely distinguishable 

 from them. It is a mere corollary or necessary inference. It is 

 modest, narrow, but also in the same proportion certain. Then 

 the group of explained facts becomes wider and wider, the laws 

 more and more general, and the theory more daring (but in the 

 same proportion also perhaps more doubtful): until it may at last 

 include the Cosmos itself in its boundless but shadowy embrace. 



Now in this gradual approach toward perfect knowledge, there 



