376 PACIFIC I1SLAN DIS. 
gress of the cone, it will keep the centre feldspathic below a short 
distance from the summit; though not to so great a distance as to 
preclude the possibility of summit eruptions of feldspathic rock at cer- 
tain periods of unusual convulsion or violent activity. In the boiling 
pools of Kilauea, the light frothy scoria is the more fusible part of the 
lava brought like a scum to the surface; at the same time, as stated, 
it is mostly a silicate of iron.* It is brought up because it is the most 
liquid part of the material, in the diminished heat near the surface. 
This is precisely a parallel case with that supposed in the great cen- 
tral vent, and we believe, therefore, that with such facts, we may 
again say we have thus far made no unwarranted assumption. 
3. The return current of lavas, if such there be, must arise from the 
liquid flowing in, to some extent, on either side, to supply the place 
of the ascending current; and this action would imply the necessity 
of a descent of the lavas of the surface, such as appears to be wit- 
nessed in the great boiling lake of Kilauea. The more basaltic por- 
tions would, therefore, constitute this exterior descending part. Thus 
a feldspathic centre and basaltic flank eruptions are a result of one 
and the same process. 
But it does not necessarily follow that the lavas of fissure eruptions 
have, in all instances, been thus derived. For they may proceed, to 
a great extent, from portions of the liquid lavas, away from the direct 
centre of the mountain, which have not experienced this separating 
process, inasmuch as lavas so situated would not undergo this separa- 
tion, unless there was a chance for the vapours to ascend and escape, 
and thereby establish a current. 
In conformity with these principles, we should find comparatively 
little chrysolite in the crater lavas of Kilauea, for the reason that this 
ferruginous magnesian silicate is a very difficultly fusible mineral. 
It does not fuse at all in the flame of the common blowpipe. It is a 
fact that although common in the scoria, it occurs on the whole but 
sparingly within the crater, compared with what is found in the re- 
sults of some fissure ejections, (p. 204.) ‘Thus the magnesia of the 
lava, or a large part of it, is engaged in combination and used up, as 
we may say, deep below. It may sometimes be wholly so, leaving 
* This glassy scoria, the composition of which is given on p. 200, appears to be a 
silicate of iron, together with the elements of a portion of augite and soda feldspar. One 
variety, the dark, afforded a large proportion of soda (21 per cent.), while the other had 
nearly the composition of an iron augite. 
