514 NEW ZEALAND SOUND (AND LAKE) BASINS, 



to base-level along their lower portions, with production of huge 

 basins thousands of feet deep below that level on certain steeps 

 and at many canon convergences, a necessity from gravitative con- 

 siderations, as can be observed by comparison with the action of 

 ordinary stream floods in determining their channel basins; with 

 that of storm waves as forming rock platforms below sea-level; 

 or as may be ascertained simply by a study of mechanics. 



(6) Similarly for corrasion in homogeneous unstable structures. 



(c)* The same reasoning holds also for a geologic complex. 

 The important point to remember in this case is that, since a 

 stream must preserve a slope from head to mouth, a strong belt 

 of rock will check vertical corrasion in weak structures higher 

 upstream. During this period the weak structures will suffer 

 wholesale destruction by lateral cutting and more or less com- 

 plete local planation ensue. f This, however, is merely a local 

 effect, for the existence of the canon stage in the lower stream 

 rocks is at most a very youthful phase of plateau reduction. 



CONCLUSION. 



Therefore, in mountains or plateaus formed of homogeneous or 

 complex structures, a plain of erosion can be formed only after 

 the transitional grade has been reached and this will, even for 

 weak streams, show a very slight fall to base-level. The Peneplain, 

 or plain of erosion, will therefore be developed practically at 

 base-level. 



LITERATURE. 



Andrews, E. C. — (a) An Outline of the Tertiary History of New England. 



Records Geological Survey N. S. Wales, Vol. vii. pp. 140-216, pls.xxxi.-xli. 



1903. 

 (b) Preliminary Note on the Geology of the Queensland Coast with 



References to the Geography of the Queensland and N. S.Wales Plateau. 



Proc. Linn. Soc. N. S. Wales, 1902, Vol. xxvii. p. 146. 



* The case of planation in horizontal strata is dealt with in the main paper. 

 t In this way a stream valley is often observed to be youthful near its 

 mouth and mature along its upper portions. The terms refer to developmental 

 stages only, not to actual age. 



