CHAP. XXXVI 
STRUCTURES OF THE BASALTS 
from an original variety in the relative abundance of steam-cavities. The 
origin of such nodular or pillow-like blocks has been already referred to at 
pp. 26 and 193. Another singular instance occurs at the foot of the 
outlier of Fionn Chro (Fig. 360), in the island of Rum. A conspicuous band 
underlying the basalts there might readily be taken for a basalt-conglomer- 
ate. Rut in this case, also, the apparent matrix is found to be amygdal- 
oidal, and the rounded blocks are really amygdales, sometimes a foot in 
length, filled or lined with quartz, chalcedony, &c, 
A somewhat different structure, in which, however, the appearance of 
volcanic breccia or agglomerate due to explosion from a vent is simulated, 
may be alluded to here. The best instance which I have observed of it 
occurs at the south end of Loch-na-Mna, in the island of Eigg, within a 
basalt which is remarkable for a streaky flow-structure. On the weathered 
faces the streaky layers may be observed to have been broken up, and their 
disconnected fragments have been involved in ordinary basalt wherein this 
flow-structure is not developed, while large blocks and irregular masses are 
wrapped round in a more decomposing matrix. There can be no doubt 
that in such cases we see the effects of the disruption of chilled crusts, and 
the entanglement of the broken pieces in the still fluid lava. 
It is a common belief that the filling in of the steam-cavities has taken 
place long subsequent to the volcanic period, by the slow percolation of 
meteoric water through the rock. I believe, however, that at least in some 
cases, if not in all, the conversion of the vesicular lavas into amygdaloids was 
effected during the volcanic period. Thus it can be shown that the basalts 
which have been disrupted by the gabbros and granophyres were already 
amygdaloids before these basic intrusions disturbed them, for the kernels 
of calcite, zeolite, etc., have shared in the general metamorphism induced in 
the enclosing rock. Again, the blocks of amygdaloid contained in the 
agglomerates of the volcanic series are in every respect like the amygdaloidal 
lavas of the plateaux. It would thus seem that the infilling of the cavities 
with mineral secretions was not merely a long secular process of infiltration 
from the cool atmosphere, but was more rapidly completed by the operation of 
warmer water, either supplied from volcanic sources or heated by the still 
high temperature of the cellular lavas into which it descended from the 
surface. 1 
4tli. Banded or stratiform lavas, consisting of successive parallel layers 
or bands which weather into projecting ribs and flutings. The deceptive 
resemblance to sedimentary rocks thus produced has no doubt frequently led 
to these lavas being mistaken for tuffs. As I have recently found them to 
be much more plentiful than I had supposed, a more detailed description of 
them seems to be required. 
1 Professor J. D. Dana, originally an advocate of infiltration from above, subsequently 
supported the view that the kernels of amygdaloids were filled in by the action of moisture 
within the rooks during the time of cooling. — Amer. Jouru. Sei. ser. 3, vol. xx. (1880), p. 331. 
Messrs. Barker and Harr have demonstrated that the Lower Silurian vesicular lavas of the Lake 
district had already become amygdaloids before the uprise of the Shap granite. Quart. Journ. 
Geol. Soc. vol. xlix. (1893). 
