ORIGIN OF LITHOLOGICAL FEATURES. 373 
terior and circumferential portions, consist of basaltic rocks: at the 
same time, the former have usually a solid, unstratified character. 
We refer to the preceding pages for the facts,* merely instancing 
here, as regards the feldspathic axis, the single case of Mount Loa, in 
which there is clinkstone at the very summit, while around the 
slopes, wherever below, the rocks are exposed in sections, and when- 
ever recent eruptions take place, there is nothing but some variety 
of basalt or basaltic lava. 
The solidity of the portions of the mountain about the axis, is ex- 
plained on the ground of the fluidity or great heat of this axial portion, 
during the action of the volcano. But what has separated the feld- 
spar, iron, and augite, that constitute the basaltic rocks, and left nearly 
pure feldspar alone at the centre? 
The only points of difference in the characters of the minerals on 
which this remarkable fact can in any way depend are the following : 
1. The different specific gravity of feldspar and augite, the latter usu- 
ally having its specific gravity as high as 2-9 to 3-2, while that of the 
former is only 2-4—2-5, or one-sixth less. 2. ‘Their fusibility, augite 
and basalt being a little more fusible than feldspar and feldspathic 
rocks. 3. In composition, basalt contains generally about ten per 
cent. more of oxyd of iron, and ten per cent. less of silica. 
The greater age of the feldspathic rocks, (although it may be a 
general truth,) cannot be in itself an explanation. There is reason to 
believe that true scoria, or vesicular basaltic lavas, might have formed 
from subaerial eruptions in the earlier periods of our globe, as at pre- 
sent, if the same amount of feldspar and augite were present, the 
same degree of atmospheric pressure, and not so high a temperature 
of the air as to influence very decidedly the rate of cooling. This 
* M. von Buch, in his account of Teneriffe, speaks of high cliffs of trachyte covered 
with basalt, and mentions also the occurrence of trachyte at the summit; and he con- 
cludes that the crater was originally trachytic, and more recently basaltic. Similar facts 
were observed at the Gran Canary. He speaks of an elevated peak of trachyte charac- 
terizing the different centres of volcanic action. M. von Buch also remarks that 'Ten- 
eriffe and Palma are in the same line trending west-northwest and east-southeast, and 
that this has determined the trachytic eruption, while Fuerteventura and Lancerote, as he 
states, have no trace of trachyte. 
There are other cases, as at St. Helena, where feldspathic eruptions have taken place 
subsequent to basaltic, and flowed in streams ; though the above represents the facts as 
they usually occur,—feldspar constituting the interior, and when in layers, forming the 
lower layers of the mountain. 
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