564 T. Mellard Reade — British Geology. 



roughly speaking, found to be proportionate to the original mass of 

 the deposits out of which they have been fashioned by earth forces. 

 If these facts stood alone they might be considered nothing more 

 than curious coincidences. If, however, we cast our eyes abroad 

 to the great continents we find that similar principles hold good, 

 and that the mountain massives are related the world over to the 

 thickness and volume of the deposits out of which they have been 

 fashioned. Thus, the Alps, the mountains of the Caucasus, the 

 Himalayas, are Tertiary structures; the Appalachians and Urals, 

 Carboniferous ; and these mountain chains are constructed of 

 enormous thicknesses and volumes of sedimentary rocks. 



Expansion and Contraction the cause of Folding and Faultings. 



We may well ask ourselves why this relation between volume of 

 sediment and greatness of disturbance should be so constant, and 

 any theory of mountain genesis must necessai'ily explain these 

 associated facts. The once favourite hypothesis which accounts for 

 them by a shrinkage of the nucleus of the earth and the closing in 

 of the non-shrinking crust upon it, and consequent folding by 

 tangential pressure, fails to explain the constancy of the connection 

 of great thicknesses of sedimentary rocks with the evolution of 

 mountain ranges. 



Neither does the principle of isostacy so insisted upon by 

 American geologists explain the compression, folding, and building 

 up of great masses of sediment into mountain ranges. On the 

 principle of isostacy, it must be obvious to anyone possessing even 

 a rudimentary acquaintance with mechanics that the sinking of 

 the bed of the seas on which great deposits are accumulating, and 

 to some extent a rise of surrounding land, may be explained, but 

 not the lateral compression and elevation of the sediments themselves 

 into mountain ranges. 



Where, then, are we to look for the agency constantly associated 

 with the deposit of great volumes of sediment which is capable of 

 eventually upheaving them from below the sea-level, and by lateral 

 compression and folding throwing them into mountain chains ? 



Again, when after the lapse of lengthened periods of geological 

 time, denudation has cut away and removed into the sea large 

 masses of elevated land, what agency is it that causes it to shrink 

 and become traversed by great lines of faulting ? 



It appears to me now, even more vividly than it has done in the 

 past, that the only agency with which we are acquainted constantly 

 associated with sedimentation and denudation, and capable of these 

 enormous dynamical effects, is change of temperature: that expan- 

 sion by increase of heat is the cause of the folding, compression, and 

 upheaval of rocks, while loss of heat and consequent shrinkage is the 

 cause of the earth fractures known as normal faults. This principle 

 I explained fully in 1886 in my " Origin of Mountain Eanges " ; 

 since then the theory has been subjected to much criticism, ranging 

 from a questioning of fundamental principles down to a minor 

 examination of small details. 



