Alpine Valleys and Alpine Lakes. 95 



3. The process of cooling would be continuous and uniform, 

 subject only to trifling local variations arising from the unequal 

 accumulation of sedimentary strata over particular areas, and 

 the unequal conductive power of certain portions of the crust. 



4. No substance is absolutely rigid ; and there is no reason 

 to doubt that amorphous masses of mineral matter, such as 

 constitute all rocks, are capable, under adequate force, of a con- 

 siderable amount of flexure. 



5. From a very early period in the earth's history, dating 

 from the first appearance of water on the surface, if not earlier, 

 inequalities in the outer surface, arising from denudation in 

 some areas, and the accumulation of sedimentary strata in 

 others, must have begun to exist, and have been continually 

 extended and varied ever since by the action of the various 

 causes that have brought the crust to its present condition. 



It is conceivable, and even probable, that during some con- 

 siderable period of the earlier history of the earth, the amount of 

 contraction from cooling in successive concentric shells may 

 have continued to be approximately proportional to their distance 

 from the centre of the spheroid, in which case no mechanical 

 forces would be called into piay that would tend to alter the 

 form of the outer surface. The conditions were altered from the 

 time when the outer crust, having approached to a constant 

 temperature, ceased to keep pace with the contraction of the 

 interior, and I ask the reader to consider the necessary effects 

 of that change. 



To fix our ideas, let us take an area of 500 miles squared, and 

 assume that during a certain period (which would be of enor- 

 mous length if counted by years) the radius of the earth had 

 contracted through cooling by 1200 yards, equal to about a 

 6000th part of its own length. Using round numbers, which 

 suffice to illustrate the argument, the result of this change will 

 have been to force the outer crust of the area under considera- 

 tion to occupy a space less by 150 yards in every direction than 

 before the contraction. If the crust were of uniform rigidity 

 throughout the same area, and also in the adjoining regions, the 

 result must be a general crushing and crumpling of the surface; 

 but we have taken it for a certainty that in reality the condition 

 of things has been otherwise, and that lines of least resistance 

 have existed which must have determined the yielding of the 

 crust in one direction rather than another. Let us assume that 

 in the regions adjoining the given area the preponderating 

 direction in which the crust had already yielded was N. and S., 

 in which case the pressure transverse to that direction will have 

 been diminished, and therefore that, during the given period 

 within the same area, the forces acting on the crust must be 



