52 OBSERVATIONS ON THE CAUSES OF ELEVATION, ETC. 



word mountain- making in preference to mountain-building, for 

 the latter term is more appropriately restricted to peaks and 

 cones built up by volcanic agency), it must be confessed that 

 whatever mountain chains were formed in the earliest geological 

 periods they have been largely obliterated in later times. This 

 is becoming more evident as geological observation extends ; 

 for it is now well established that in Asia, South America, 

 North America, and in Europe, the great era of mountain 

 making was during the Tertiary period, and even in Austral- 

 asia it is probable that its principal Alpine ranges were 

 inconspicuous ridges during the early part of the Mesozoic 

 period. Still the intensity and universality of the elevations 

 then produced demand a corresponding universality of cause, 

 or causes. A considerable number of hypotheses at different 

 times have been proposed by eminent investigators, all of 

 which commend themselves in some particulars, but all of 

 which, taken separately, involve difficulties which at present 

 appear to be insurmountable. Perhaps the truth lies in 

 several, and, at least, does not demand the rejection of one 

 cause because another seems to answer all the conditions when 

 confined to a particular case. It appears to me, however, 

 that there is one which may be well considered as the govern- 

 ing or mother cause to two or three of the rival hypothetical 

 causes ; the latter being consequents standing in the relation 

 of child to parent rather than absolutely independent causes. 

 The parent cause, in my opinion, appears to be the Contraction 

 Theory so intimately associated latterly with the name of 

 Eobert Mallet. 



Contraction Theory. 

 If our planet has been steadily losing heat by radiation into 

 space a corresponding diminution in its volume must also be 

 looked for ; for cooling is implied in contraction. A succinct 

 account of the effects which Mr. Eobert Mallet demonstrated 

 might be expected from the secular cooling of our globe is given 

 as follows by Dr. G-eikie : — "According to Mallet the diameter 

 of the earth is less by 189 miles since the time when the 

 planet was a mass of liquid. But the contraction has not 

 manifested itself uniformly over the whole surface of the 

 planet. The crust varies much in structure, in thermal resist- 

 ance, and in the position of its isogeothermal lines. As tho 

 hotter nucleus contracts more rapidly by cooling than the 

 cooled and hardened crust the latter must sink down by its 

 own weight, and in so doing requires to accommodate itself to 

 a continually diminishing diameter. The descent of the crust 

 gives rise to enormous tangential pressures. The rocks are 

 crushed, crumpled, and broken in many places. Subsidence 

 must have been the general rule, but every subsidence would, 

 doubtless, be accompanied with upheavals of a more limited 



