132 E. C. ANDREWS. 



a general height at its centre of several hundreds of 

 feet, provided the divide be situated at a considerable 

 distance from the sea. 



(2) Computations of the age of the earth based upon the 

 estimated rate of denudation at the present day. 



It is common to find the age of the earth's sedimentary 

 record based on the assumption that denudation of the land 

 occurs at a fairly uniform rate. 1 



The study of erosive processes does not bear out such 

 assumption for the following reasons : — 



(a) The mountains of the present day are doubtless com- 

 parable in size with those of any Post- Archaean Age. 



(b) The mountain valleys of the present day are quite 

 youthful, and their loads are enormous by reason of their 

 great transporting and corrasive power. Such transporting 

 and corrasive power is related in a rapidly increasing 

 geometrical ratio to the simple increase of stream velocity. 



(c) Mountains are only transient forms in a landscape 

 during the cycle of erosion necessary for the production of 

 a peneplain. 



(d) The time necessary to reduce a continent or plateau 

 from a height of 1,000 feet to a peneplain of 500 feet in height 

 at its centre involves practically the same length of time 

 as the reduction of a plateau 20,000 feet in height to a 

 similar stage when all other things are equal. This is 

 simply an application of the known laws of geometrical 

 decrease of corrasive and transporting powers upon the 

 great reduction of stream channel and land slope. 



(e) Many great peneplains or surfaces or erosion have 

 been formed at various periods in the earth's history. 



It would thus appear that estimates of the earth's age 

 based on the assumption of uniform denudation are 

 altogether too small. 



A. R. Wallace, Island Life, 1892, pp. 210-240. 



