538 GEOLOGY 



have carried on their discussions as if one of the suggested causes 

 must be selected to the exclusion of the others. 



As a matter of fact, almost every complex geological phenome- 

 non has not a simple, but a composite, explanation. To illustrate, 

 in Chamberlin and Salisbury's text-book of geology it is stated 

 that the explanation of volcanism may be given upon the assump- 

 tion that the lavas are original; or, second, on the assumption 

 that the lavas are secondary. Under the first assumption it is sug- 

 gested (1) that lava outflows from a molten interior, and (2) that 

 lavas flow from molten reservoirs. Under the second assumption 

 it is suggested that lavas may be assigned (3) to the reaction of water 

 and air penetrating to hot rocks, (4) to relief of pressure, (5) to melt- 

 ing or crushing, (6) to melting by depression, and (7) to the outflow 

 of deep-seated heat. 1 At the close of the discussion it is said that 

 these hypotheses "must be left to work out their own destiny." 2 

 I fear many will make the inference, although I have no idea that 

 the authors so intended, that one among these hypotheses will be 

 victorious in the struggle for existence and the others totally over- 

 thrown. My point in this connection is that the two main supposi- 

 tions, and all of the hypotheses under them, may be true in part; 

 that these various explanations are not necessarily exclusive of one 

 another, but may be supplementary. When we have a quantitative 

 discussion of the probable effects which may be expected from each 

 of the causes suggested, we shall have some idea of their possible 

 relative importance. For my own part I have no doubt whatever 

 that volcanism is to be explained by some combination of the seven 

 causes mentioned, with doubtless other causes which have not yet 

 been suggested, rather than by a single cause. As soon as it is appre- 

 ciated that to explain a complex phenomenon several causes are 

 usual, if not invariable, rather than exceptional, it becomes plain 

 that their relative importance should be determined, and this can be 

 done only by quantitative methods. 



The Individual Problems of Geology 



Thus far we have been considering the problem of geology as a 

 general one. The subject assigned, "The Problems of Geology," 

 might imply a treatment of the particular problems at present being 

 considered by geologists. For an address this interpretation of the 

 subject is impracticable. Adequately to discuss one of the unsolved 

 problems of geology from the point of view advocated would require 

 a monograph. Not only is it impossible to discuss unsolved problems 



' Chamberlin and Salisbury, Geology, vol i, Processes and their results, pp. 595- 

 uu^j iyo4. 



2 Ibid., p. 602. 



