34 



TUTIRA 



sort of water sandwich, the sands and grit being carried off particle by 

 particle between the floor of marl and the ceiling of humus. In process 

 of time the sharp pumice grit chisels out of the former a minute irregular 

 bed. It deepens into a tiny hidden runnel ; at last there is created a 

 subterranean stream, or, in shepherd's phrase, an "under-runner." Its 

 course is at first unseen, then, as through process of time its channel is 

 gouged out, and as the carpet of humus fibre gives way at irregular 

 intervals, great rents and holes betray its presence. The bed continues 

 to deepen, the carpet of turf falls in more and more, until finally the 

 under-runner becomes an open rivulet. Ancillary effects now become 



Under-runner, the roof of which has here and there given way, 



prominent, along each edge of the rivulet a secondary process of 

 subcutaneous attrition is set up. Subsidiary under-runners become 

 established at right angles to the stream, and in time laterals also to 

 the latter. The stream increases in depth, the angle of inclination of 

 the slope on either side grows more acute. In time this triple process 

 of erosion, still subcutaneous, still veiled by the carpet of rooty humus, 

 saps up to the containing walls of the interstice, and now for the first 

 time on Tutira we see a possibility of a widening as well as of a deepening 

 of the gap. The containing walls are not of conglomerate as in central 

 Tutira, nor are they of limestone and sandstone as in west Tutira. They 



