igoS.] 



THE PHYSICS OF THE EARTH. 245 



great changes of form produced by a different process and at a much earlier 

 period. We shall speak of this later. 



" 5. Alps. — 'Sir. Judd has recently shown that the region of the Alps, 

 during the whole ]\Iesozoic and Early Tertiary, was a marginal sea bottom, 

 receiving sediments until a thickness was attained not less than that of the 

 Appalachian strata. At the end of the Eocene these enormously thick sedi- 

 ments were crushed together with complicated foldings and swollen upward 

 to form these mountains and afterward sculptured to their present forms. 



" The same may be said of the Himalayas and nearly all other moun- 

 tains. We may, therefore, confidently generalize, and say that the place 

 now occupied by mountain-ranges have been previous to their formation, 

 places of great sedimentation, and therefore usually marginal ocean bottoms. 

 In some cases, however, the deposits in interior seas or mediterraneans have 

 yielded in a similar way, giving rise to more irregular ranges or groups of 

 mountains." . . . 



" Why thick Sediments should be Lines of Yielding. — Admitting, then, 

 that mountains are formed by the squeezing together of lines of very thick 

 sediments, the question still occurs. Why does the yielding take place along 

 these lines in preference to any others? This is a capital point in the 

 theory of mountain formation. The answer is as follows : We have already 

 seen (p. 231) that accumulation of sediments causes the isogeotherm to rise 

 and the interior heat of the earth to invade the lower portion of the sedi- 

 ments with their included waters. Now- this invasion of heat in its turn 

 causes hydrothermal softening or even fusion, not only of the sediments, 

 but also of the sea-floor on which they rest. Thus a line of thick sediments 

 becomes a line of softening and therefore a line of weakness, and a line 

 of yielding to the lateral pressure, and therefore a line of mashing together 

 and folding and upswelling — in other words a mountain-range. As soon as 

 the yielding commences we have an additional source of heat in the crush- 

 ing itself. In addition to this, upheaval by lateral crush by the tendency 

 to arch the strata would produce relief of gravitative pressure, and there- 

 fore fusion (p. 103). It follows from this that there is or was beneath 

 every mountain a line of fused or semi-fused matter. This we will call 

 the sub-mountain liquid. This by cooling and solidification becomes a mcta- 

 niorphic or granitic core, which by erosion forms the metamorphic or granitic 

 axis and crest of many great mountains" . . . (pp. 271-2). 



" Cause of Lateral Pressure. — We have thus proved that the immediate 

 cause of the origin and the growth of mountains is lateral pressure acting 

 on thick sediments, crushing them together and swelling them up along the 

 line of great thickness. But still the question remains, What is the ultimate 

 cause, /. e., the cause of the lateral pressure? This, as we have already said, 

 lies still in the domain of doubt and discussion, but the view which seems 

 most probable may be briefly stated as follows : 



" In the secular cooling of the earth there would be not only unequal 

 radial contraction, giving rise, as shown on page 175, to continents and 

 ocean-basins, but also to unequal contraction of the exterior as compared 

 with the interior. At first, and for a long time, the exterior would cool 



