chap. III 
STRUCTURES OF LAVAS 
17 
from the removal of their constituent ingredients by the solvent action of 
'vater on the surrounding rock. And as amygdaloids are generally more 
decayed than the non- vesicular lavas, it has been generally believed that the 
abstraction of mineral material and its re-deposit within tlie steam-vesicles 
have been due to the influence of meteoric water, which at atmospheric 
temperatures and pressures has slowly percolated from the surhice through 
the cellular lava, long after the latter had consolidated and cooled, and even 
*frter volcanic energy at the locality had entirely ceased. 
Examples, however, are now accumulating which certainly prove that, in 
®ome cases, the vesicles were filled up during the volcanic period. Among 
the Tertiary basalt-jdateaux of the Inner Hebrides, for instance, it can be 
®frown that the lavas were already amygdaloidal before the protrusion of the 
Flo. 2. 
Elongation and brandling of .steam-veside.s in a lava, Kilniiiian, 
than natural size. 
Lsle of Mull, a little less 
gabbros and granophyres which mark later stages of the same continuous 
^olcanic history, and even before the outpouring of much of the basalt of 
plateaux. Not improbably the mineral secretions were largely due to 
I'm of hot volcanic vapours during the eruption of the basalts. 
's subject will be again referred to in the description of the Tertiary 
’^’olcanic series. 
V esicular structure is more commonly and perfectly developed among 
lavas which are basic and intermediate in composition than among 
ose which are acid. 
Idle the existence of a highly vesicular or scoriaceous structure may 
honerally be taken as proof that the rock displaying it flowed out at the 
®'irtace as a lava, other evidence pointing to the same conclusion may often 
gathered from the rocks with which the supposed lava is associated. 
''’OL. I c 
