108 TRANSACTIONS OF THE [ Sess. Lxxv. 
called locally, occupied by vegetation, even, perhaps, by a 
closed covering, and quite abandoned by the river proper, 
though at their junction with the present flood-plain they 
end in a ragged or undermined face of bare stones. 
The streams are not confined to any special part of the 
bed, but generally every few kilometres the main stream 
crosses from one side to the other of the valley, the point 
of departure being very often a rocky bluff against which 
it abuts, there deepening its bed and depositing its load of 
shingle on the lower side of the bluff. Thus the river-flat, 
lying below such a rocky point washed by a non-shifting 
main stream, is the most stable portion of the valley, and 
bears the oldest plant-covering. 
The Rakaia river-bed here dealt with shows excellently 
all the features detailed above. The two glaciers from 
which the source-branches of the river issue are covered 
with an astonishing quantity of moraine (see Plates IX. 
and X.) for a distance of several kilometres from their 
terminal faces; there are numerous torrents bringing 
shingle from the débris-fields, and distributing it, fan-like, 
on the main river-bed; there is a wide expanse of bare 
shingle traversed by a network of unfixed anastomosing 
streams subject to frequent floods; there is abundance of 
dry stony ground, and there are islands and flats of every 
degree of stability. The stable flats are themselves watered 
by streams, which may be connected with the main river 
or have their origin on the adjacent heights, but these 
streams are generally much less rapid than the branches 
on the flood-plain proper, and may contain more or less 
plant-life in consequence. The main stream at about 
4 km. from its source in the Lyall glacier flows all in one 
channel (see Plate IX.) except at flood-time, and generally 
rushes with such violence that the sound of the great stones 
it carries along, grinding one against the other, may be heard 
very plainly even from the slopes 600 m. above, or more. 
The river-water contains a very great amount of fine 
glacial mud, so that it is quite milk-white in colour, 
consequently the retreating waters after a flood leave 
much fine silt deposited between the stones, or coating 
the floors of hollows with a layer of varying thickness. 
