Page:The Journal of geology (IA journalofgeology11893univers).pdf/136

But there still remains the question: "What is the cause of the lateral pressure?" The answer to this question constitutes the physical theory.

Thus far I suppose there is little difference of opinion. I have only tried to put in clear condensed form what most geologists hold. But henceforward there are the most widely diverse views and even the wildest speculations. But let us not imagine, 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 investigations. 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 pressure, though still of extreme interest, is certainly of less immediate importance in guiding investigations.

The most obvious view of the cause of lateral pressure refers it to the interior contraction of the earth. This may be called the

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