420 E. O. HOVEY WALLIBU AND RABAKA GORGES 



Furthermore, Curtis's figure represents the big fragments and bowlders 

 as being concentrated in the bottom of his V-shaped gorge, to form a 

 sort of accumulating region or channel for underground waters. This 

 theory assumes that the big masses were thrown into the gorge first and 

 the gravel and sand afterward, which can not have been strictly the case. 

 Material of all sizes was thrown out together from the crater, and some 

 sorting would naturally have followed deposition from clouds suspended 

 in the air, but as a matter of fact the natural sections of the ash beds 

 show that masses as much as 30 to 50 centimeters across were relatively 

 scarce and scattered irregularly throughout the ash beds. Where they have 

 reached the bottom of the gorge it has been mostly, at least, through being 

 left behind on the removal of the finer material. The filling of the 

 gorges was done for the most part by the rolling, sliding debris-avalanches 

 that formed part of each great eruption cloud, in which there was little 

 opportunity for assorting material according to size. Water sinks and 

 flows more rapidly through coarse material than it does through fine, but 

 the exposed sections of the new ash show the presence of enough irregular 

 lenses of gravelly and bowldery material and clean sand to account for the 

 repeated access of steam and rain water to the heated interior of the beds 

 which was evidenced by the frequent violent explosions that were noted 

 by all observers. With the gradient (3 to 4 degrees) still obtaining for 

 the surface of the floodplain of the lower reaches of the Wallibu, the 

 rapid stream 4 or 5 meters wide and about 1 meter deep that pours 

 through the flat or possibly U-shaped gorge (rock-walled as to one 

 side) of the upper reaches, ending about 3 kilometers from the coast, 

 soon loses itself in the sand and gravel and pursues an underground 

 course to the sea. The length of this subterranean flow varies up to 

 about 2 kilometers, depending upon the volume of water in the stream. 

 In June, 1908, it seemed to require a heavy shower of at least an hour's 

 duration to bring the river down on its floodplain to the sea. After the 

 stream bed became thoroughly saturated with water, comparatively gentle 

 showers would keep the river running on the surface for many hours, 

 perhaps for days. The tendency of the less important showers is to accu- 

 mulate debris along the lower middle reaches of the stream bed, increasing 

 the gradient of the flood-plain. 



In the hollows of the original surface of the new ash bed, where its 

 nature did not permit rapid percolation, but was favorable to the accumu- 

 lation of water, pools were formed. These gradually enlarged until they 

 coalesced, and the stream, struggling down from the hill slopes, found its 

 way from one pool to another with varying vicissitudes. Occasionally a 

 secondary eruption would throw a dam across the stream, impounding the 



