562 THE JOURNAL OF GEOLOGY. 



But there still remains the question : "What is the cause of 

 the lateral pressure?" The answer to this question constitutes the 

 pliysical theory. 



Thus far I suppose there is little difference of opinion. I 

 have only tried to put in clear condensed form what most geolo- 

 gists hold. But henceforward there are the most widely diverse 

 views and even the wildest speculations. But let us not imag- 

 ine, on that account that we have made no progress in the 

 science of mountain-origin. The formal theory already given is 

 really for the geologist by far the most important part of the 

 theory of mountain-origin. For I insist that for the geologist 

 formal theories are usually more important than physical theories 

 of geological phenomena. That slaty cleavage is the result of a 

 mashing of strata by a force at right angles to the cleavage- 

 planes, is of capital importance to the geologist, for it is a 

 guide to all his investigations. To what property of matter 

 this structure is due is of less importance to him, though of 

 prime importance to the physicist. That the phenomena of the 

 drift is due to the former existence of a moving ice-sheet is the 

 one thing most important to the geologist, guiding all his inves- 

 tigations. Whether this ice-sheet was caused by geographical or 

 astronomical changes is a question of wider but of less direct 

 interest to him. So in the case of mountain ranges, the most 

 important part of the theory is their origin by lateral pressure 

 under the conditions given above. The cause of the lateral pres- 

 sure, though still of extreme interest, is certainly of less immedi- 

 ate importance in guiding investigations. 



PHYSICAL THEORIES. 



The most obvious view of the cause of lateral pressure refers 

 it to the mterior co?itractiou of the earth. This may be called the 



" CONTRACTIONAL THEORY." 



This theory is so well known that I will give it only in very 

 brief outline. It assumes that the earth was once an incandescent 

 liquid and has cooled and solidified to its present condition. At 

 first it cooled most rapidly at the surface and must have fissured 



