112 president's address — SECTION c. 



blocks " (Cotton, 1917). During degradation the filling from 

 some troughs may be removed, and the underlying rock exposed. 

 The local base-level of erosion from the covering strata within a 

 trough must depend on the level at which the streams escape from 

 the basin across the barriers of the more elevated blocks of the 

 under-mass. The streams escaping across these barriers must be 

 superimposed in a certain sense. Commencing as streams on the 

 blocks as they were originally dislocated, they have cut down 

 into the covering strata until they were forced to notch the under- 

 mass, but Cotton has pointed out that they so frequently cross the 

 barriers of under-mass, in gorges cut where the general surface of 

 the planed surface of the under-mass shows a down-warped depres- 

 sion, as to indicate that the streams must have existed formerly at 

 these warped depressions as consequent streams on the overlying 

 covering mass, and have since become established by deep erosion. 

 Thus they are " superimposed " ccnsequeiit streams, rather than 

 " antecedent " streams, for they are not necessarily older than 

 the main dislocation of the crust in their drainage area. But if a 

 stream escape from a tectonic basin by a gorge cut in a depression 

 in the rim of the basin that is not the deepest deipressioii, it must 

 follow that, when the original consequent stream escaped from the 

 gap, it was the deepest depression, and that subsquent movem'ent 

 has taken place by which another portion of the rim has become 

 more deeply depressed, while the river has remained incised in the 

 gap from which it first escaped. Such a type of river Dr. Cotton 

 terms " anteconsequent." and he has indicated some examples of 

 this (1917), and pointed out that somewhat similar conditions have 

 been noted by Davis in Herzegovina. (See also Speight, 1918.) 

 These useful conceptions Cotton (1917) has shown to have a very 

 wide application throughout New Zealand. In particular, in Cen 

 tral Otago, there is a wide area composed of a chain of broad 

 tectonic depressions interspersed with elevated masses of older rock, 

 schist or greywacke (which themselves appear to be made up of a 

 mo«aic of fault-blocks separated by a system or systems of rela- 

 tively ancient faults to which the present relief is largely or wholly 

 indifferent, except in so far as faulting on the old lines of move- 

 ment may have been revived. The relief due to movements on 

 these ancient fault -lines, together with that resulting from earlier 

 folding, had been almost or wholly destroyed prior to the deposition 

 of the over-mass or cover. Though the later faulting:, to which 

 the existing relief is due, appears to have followed the lines of 

 the older faults in some places, the displacement has generally 

 been reversed (Cotton. 1917), Many details of the crust -move- 

 ment — warping, faulting or splintering — are indicated by the 

 form of these salient blocks of the under-mass, especially where 

 they bear isolated residuals of the covering strata. fSee Fig. 8.) 



Where depressed blocks were long, narrow strips of country, they 

 now are represented by trough-like valleys such as are seen in parts 

 of North Cant.eTbury (Speight, 1918, p. 97), and. perhaps, at 



