69 
aniples of streams cutting down into their beds rather than flooding 
and building up ])lains. Where the Lawlers road crosses Poison 
Creek, there is a distinct a])pearance of stratification in the ferru- 
ginous grits seen in the sides of the creek bed. and something of the 
sort has also been noticed in the bed of the (lascoyne (S. 
Branch), near Mt. Ifgerton, and in that of the river on the track 
from Ruby Well to Wiluna. At this stage, however, we may leave 
the question of the formation of the sniiorficial material composing 
the plains to be taken up later on when some other evidence bearing 
upon it has been described an<l examined. The point of most im- 
portance just at present is that the existing rivers arc cutting dowi- 
into the plains and not l>uilding them up. Nevertheless, it is not 
over-looked or forgotten that comparatively slight movements of 
elevation and subsidence of the land as a whole may alter the grade 
of rivers from time to time, so that at one lime they' tend to fiL' 
their valleys with alluvial flood-plains and at another to cut down 
through these deposits on to the bed-rock again. It is therefore 
freely conceded that by itself no great signilicance should be at- 
tached to the fact that the existing rivers are cutting into the plains 
which they traverse, nor can their present ])hase of action be taken 
to prove that they have not themselves formed these i)lains in the 
first instance. It is when taken in connection with a number of 
other considerations that their evidence should be regarded as cor- 
roborating the marine i)lanati3n theory rather than that of river 
erosion 
r>efore leaving the subject of the existing rivers, however, 
mention should be made of a feature of several of them which may 
be of much significance in assisting us to trace the historical de- 
velopment of the [jreseiit to])Ography, namely, the numerous ‘’gaps” 
which are so noticeable in the N.W. districts. Many of these are 
descril)ed in chai)ters 1 and 2 of part VI. of Mr. Jutson’s bulletin. 
])reviously referred to, closely rescnil)ling one another in the 
general feature that rivers are found breaking through ranges of 
hills quite unexpectedly’, without any very visible reason why’ they’ 
should have been able to find an outlet through the high country 
instead of going round it. following the i)resent low ground. If 
the country on the up-river side of the hills were generally high, 
we might understand that it has been formerly higher than the 
hilly country traversed, and that the rivers have cut their way 
down through the hills simultaneously with a general lowering of 
the upper parts of their basins to a level much below that at which 
the hills were originally attacked. But the most curious point 
about the gaps which the writer has seen, is that they leave a plain 
on one side of a ridge of hills, plunge through the ridge, and 
come out on another plain on the other side, and that if the ridge is 
followed along its length the plain on the u])-river side is found 
to be practically continuous with that on the down-river side, 
round the ends of the ridges. The Shaw River has three “gaps” 
