374 PACIFIC ISLANDS. 
remark is opposed to the assumptions of many writers on igneous 
rocks, yet appears to be founded in truth; for heat and pressure, and 
slowness of cooling, the requisite material being present, are the pro- 
minent influencing causes, determining the formation of particular 
minerals or rocks ; and the existence of the constituents in the proper 
proportions, is shown in the many trap and hornblendic rocks of 
ancient date. 
Moreover, these feldspathic rocks are apparently an essential part 
of the volcanic dome, during its whole progress, as they occur along 
the centre, often to the very summit. We have no grounds for sup- 
posing such an insertion of feldspar rocks after the dome was com- 
pleted, and as little for imagining a feldspar peak (of fourteen thousand 
feet for Mount Loa), to have been first thrown up.* ‘The only con- 
clusion sustained by the facts is, that the main body of the mountain, 
and its feldspathic centre, were in cotemporaneous progress, at least 
as long as there were summit eruptions. 
May there be a separation of the basaltic material by gravity, and 
thus feldspathic eruptions take place at summit, while the lateral are 
basaltic? ‘This ingenious view is presented by Mr. Darwin. But if 
there were a simple subsiding of the ferruginous basalt, should we not 
sometimes find the phonolites graduating below into basalt, or be- 
coming more ferruginous? But this is never the case, as far as facts 
are known.t+ It is, moreover, remarkable that in Kilauea, the scoria 
* It has been frequently said that feldspathic lavas are not as liquid as the basaltic ; 
and this assumption is made in order to account for a supposed swelling up of the molten 
rock into domes. It is true that perfect liquidity requires greater heat for the former than 
the latter: but what is there in the nature of things, or what reason of any kind, for this 
limited heat at all former periods? If it be considered that viscidity in the lavas of a 
vent, necessarily occasions scoria or cinder eruptions, provided waters gain access below, 
it will be obvious that the absence of such cinder eruptions is some evidence of unusual 
liquidity. 
+ Mr. Darwin supposes that the feldspar rises in the lava as feldspar crystals, and the 
augite sinks as crystals of augite. (Volc. Islands, p. 120.) But if any thing is in fusion, 
it is feldspar or augite, (supposing these to be the constituents of the lava,) and while in 
fusion there are no crystals, as crystallization is the first step in the process of solidifica- 
tion. If there is any sinking, therefore, it must be a sinking and separation of these 
materials in the fluid state; or, at least, this must be the case with the augite, which is 
the more fusible of the two minerals, and will be the case with both at certain depths, 
wherever the temperature is that of the fusion of feldspar. The impossibility of there 
being any crystals of feldspar, will farther appear from the fact that the material of 
clinkstone is generally without a distinct crystalline texture, 
