254: C. A. Cotton — Block Mountmns in New Zealand. 



It is necessary to consider criteria for distinguishing tlie 

 types of drainage. Subsequent streams guided by weaker 

 strata in an inclined series are readily recognized, but when 

 they are guided by shear-zones, ancient fault-planes, or master 

 joints they are generally included with the insequents. Lead- 

 ing these aside, we may consider the important types of drain- 

 age in the early stages of the cycle of erosion introduced by the 

 deformation, namely, true consequents, true antecedents, and 

 anteconsequents, all possibly superposed. 



Trice Consequents. — On the highly improbable assumption 

 that deformation is simultaneous and instantaneous and without 

 contemporary erosion, it follows that consequent streams will 

 make their way down the tilted and warped surfaces and that 

 some consequent lakes will be formed. Under normal condi- 

 tions of humidity, these lakes will spill over at the lowest gaps 

 along consequent courses, which will be superposed later on the 

 structure of the older mass, gorges will be cut through the 

 higher blocks, and systems of consequent streams will be estab- 

 lished. Exactly similar drainage patterns are to be expected if 

 the deforming movements are simultaneous though not instan- 

 taneous. If the movements are sufficiently slow no lakes may 

 result, as early-formed consequent courses across the lowest sags 

 in the crest lines of the rising blocks will be continuously deep- 

 ened by corrasion. Consequent lakes considerably above base- 

 level are short-lived, so they are not likely to leave permanent 

 records of their existence. 



Anteconsequent Drainage. There is as little justification 

 for the postulate that movements have been simultaneous 

 throughout a period of deformation as there is for that of in- 

 stantaneous deformation. Movement may be well-advanced in 

 some parts of a region while other parts are as yet unaffected. 

 There may even be a rhythmic passage of waves across the 

 land surface. (In regional movements such oscillation is well 

 attested; but the present discussion is concerned with strongl}^ 

 differential as distinguished from regional movements.) 



In the case of a low-lying block surrounded by differentially 

 rising blocks, the movement of which is not necessarily simul- 

 taneous, the lowest gap in the basin rim (erosion being left out 

 of account) may not always be in the same position; and the 

 consequent outlet of a basin established during an early stage 

 of deformation and persisting by rapid corrasion may not at a 

 later stage be situated at the lowest sag in the surrounding 

 blocks. 



Antecedent Drainage. Antecedent drainage channels on a 

 surface of the kind postulated must be inherited from a simple 

 centrifugal system of subparallel streams radiating from the old 

 land or that portion of the undermass which escaped burial 



