494 Mr. T. M. Reade on the Origin of Mountain-Ranges 



manent feature of the Earth's surface. The contraction must 

 be satisfied in another way, either by breaking up into blocks, 

 faulting and subsidence extending through its substance, or 

 by vertical contraction alone, and the lengthening of the beds 

 by compressive extension due to the weight of superimposed 

 materials. Probably both these principles generally come 

 into operation together in nature. The Earth is bound to 

 retain its solidity in whatever way that may be satisfied. If 

 a rise of temperature then succeeds, the effect will be as be- 

 fore, and deformation will result, its locality being determined 

 by the line of least resistance. 



In the case of a mountain -range it will take place along or 

 parallel to its axis, and the range will receive another acces- 

 sion of bulk. 



Thus we see that the effect of alternation of temperature in 

 the Earth's crust leading to the establishment of mountain- 

 chains is cumulative. This cumulative effect of small alter- 

 nations of temperature may be seen in the ridging-up of any 

 old lead gutter, lead flat, or lead-lined bath or sink. It has 

 been likened to a " rachet " movement, which is not an inapt 

 illustration if taken with the necessary qualifications. 



Effects of Contraction. — Normal Faulting. 



Normal faults, that is faults that hade to the downthrow, 

 are the result of contraction, and are posterior to the first 

 plication. Any section of a mountain-range traversed by 

 normal faults shows the folds sheared in a way that proves 

 this. Normal faulting is, however, most prevalent in the less 

 disturbed strata that flank a range. The mountain-range, 

 pushed up by successive lateral thrusts or recurrent ex- 

 pansions acting over a great length of time and the folds 

 thrown back and further compressed by the cores of gneiss 

 and granite intruded into them, becomes a solid mass which 

 cannot be drawn back by contraction. Contraction therefore 



(has its maximum effect on the more horizontal deposits that 

 flank the range, and extend for considerable distances on 

 either side. 



As the crust of the Earth must remain solid, the condition 

 is satisfied by shearing and wedging-up by gravitation, — other- 

 wise by normal faulting. Contraction of igneous masses 

 beneath may induce this faulting in some cases, but it is not 

 a necessary condition. Cubical contraction of the solid crust 

 is sufficient. 



