32 TUTIRA 



trees, there are to this day no inequalities to vary the monotony of its 

 even blanket of dark fibrous humus. 



Our first hypothetical block was selected from the conglomerates of 

 central Tutira, where conditions are most simple and where the valleys 

 are invariably dry except for such rain as reaches them from the 

 sky. A further development of the subcutaneous drainage system 

 will again be made easily comprehensible if once more a conjectural 

 block be visualised, taken this time from the limestone ranges of the 

 west. As before, we must imagine the slope towards the east, the skin 

 of matted humus covering grit and pumice sands, the subterranean 

 soakage system, the right-angle rivulet at base, the western precipice ; 

 finally, the different stages of sag as exemplified in the history of the 

 conjectural conglomerate block of central Tutira. We can start, in fact, 

 now where we concluded then that is, with a deep sag between cliffs 

 of 10 or 20 feet. Our western valley, however, to begin with, is many 

 times the length and width of the central valley. Owing to its greater 

 area, there is a quite important rainfall reaching it from the skies. 

 It stands at a higher elevation above sea-level : there is a greater 

 fall for its drainage system. Shrinkage in the sag becomes more and 

 more pronounced, until the rock walls stand up 80 and 100 feet on 

 either side, until there comes a time when the centre of the fold under- 

 mined by the soakage system, by the flushing action of springs, which 

 now first come into account, and by rain-water falling within the gorge, 

 changes from a deep lap to an angled incline from a U, in fact, to a 

 V. The extreme point, the apex of the inverted V of humus skin, is 

 now for the first time in our story directly exposed to water. It is 

 finally worn through by the action of the running water of springs 

 supplemented by soakage of heavy rainfalls ; a brook trickles over the 

 lower portion of the sag ; a normal valley, in fact, has been formed save 

 for the impossibility of lateral expansion. 



Proceeding once more from the less simple to the least simple, the 

 reader is invited to picture a third conjectural section, on this occasion 

 from east Tutira. Once more, then, a block must be imagined showing 

 the typical eastern cant, the even upper covering of humus grit and 

 red sand, the drainage system at right angles to the tilt of the 

 slope. As, however, we find differences between the central and western 

 sections, so again we discover others now. The interstices of the east 

 are wider than elsewhere, the cachment area larger, the rainfall heavier, 



