Stony Debris Formations. 227 
of metres. Here and there jagged masses of much corroded rock jut out 
from these stone-fields but hardly break the monotone of the vast, grey even 
ni slopes which extend from the lower subalpine belt to the mountain-tops. 
Ei Gullies, often with rocky walls seam the mountain sides, their floors occupied 
= by a stream, its source the base of some great stone-field where all on a 
Be sudden water bursts forth. 
The stones themselves differ in size but the bulk are generally small, per- 
haps 5 or 6 cm long by 2 cm broad, though some may be much larger. 
Those of the upper layer are quite jöoae and, as the surface is steep, they 
‚ are liable to slide downwards, considerable reale when disturbed, moving 
en masse. At 30cm or more from the surface, the ground is more stable 
and there is generally a good deal of finer debris, sand, and even clay mixed 
with the coarser stones. Although quite dry on the surface, at a few centi- 
metres depth the substratum is damp, and deeper still ample water, but icy- 
cold, is available for plants. The special climate of the habitat depenis upon 
extreme exposure to wind; strong radiation of heat from the stones; powerful 
heating of the stones themselves and, at times, very bright light. Within the 
Space of a few hours the plants are frequently subjected to burning heat and 
considerable frost, or one hour they may be surrounded by moist air and the 
next be exposed to a strong, dry wind. Those which are evergreen bear a 
' heavy weight of snow for four months or more. Nor are occasional droughts 
' unknown. It is obvious then that the ecological conditions of shingle-slip are 
distinctly those of desert, while in addition there is marked instability of sur- 
face. This latter character has, in part, led to the occupation of the ground er 
‚hot merely by certain peculiar growth-forms but dy 26 distinet species which a 
do not occur in any other formation. er 
Near the edges of the shingle-slip, the stones are far less table to, move, in 
and there is stability sufficient for species other than those. adapted to the 
_ moving debris to settle down, so that, by degrees, the formation is trans- 
formed into fell-field. But REN er change the actnal ee ass 
 edation contributes nothing, its members are too far apart and too few 
a appreciable humus to the soil. The formation is indeed. distinet. in itelf and 
not a phase in the development of er but a definite eig the | 
origin of which is wrapped in obscurity. . 
 . Shingle-slip, in its unstable and typical EN is onfined to those mount- 
 ains of the South Island with a steppe-climate and is | ine 
ee in the North- eastern and ‚Eastern distriets. 1ere 
Hi 
Pation by neh plants. is Ber more a rtain S 
‚the North Island volcanoes are ecologically similar to Ene- 
Schist Mountains. A the a IN: i 
