152 
THE TERTIARY VOLCANOES 
BOOK VIII 
tinuously, but may have only touched its upward projecting vaults. Hence 
gaps would arise in the continuity of the dyke-material. 
The ascent of lava from a line of such separate openings along a fissure 
would necessarily involve lateral as well as vertical movements in the 
molten mass which would be forced along the open rent until the several 
streams united and filled it up. We might therefore expect somewhere to 
find instances of flow-structure in the dykes pointing to these movements. 
I have already referred to the lines of amygdales frequently noticed in 
dykes, especially towards the centre. Occasionally these steam- vesicles may 
be observed to be drawn out in one general direction indicative of the 
trend of motion of the molten rock. 
Some of the best examples of this feature which have come under my 
observation occur among the trachytic dykes of the south-east coast of Skye 
between Kyle Bhea and Loch na Daal, where they have been mapped and 
carefully investigated by Mr. Clough, who has conducted me over the sec- 
tions. In some of these dykes, as already narrated, the marginal portions 
display a finely spherulitic structure, the small pea-like spherulites being- 
grouped into fine ribs or rods. It is also observable that the steam- vesicles 
which may retain their spherical forms in the centre are elongated in the 
same direction as the rows of spherulites. Where this lineation is 
developed vertically, it no doubt points to the vertical ascent of the lava 
between the two walls of the fissure. 
But in other examples, the elongation is nearly horizontal, and between 
the two positions Mr. Clough has registered many intermediate trends. It 
would thus appear that in some places the lava has certainly flowed laterally 
between the fissure-walls. Moreover, the trend of the spherulitic rods and 
of the amygdales is found to vary in closely adjoining planes at different 
distances from the margin, as if after the outer portions of the dyke had 
consolidated into position, there was still movement enough to drag the rows 
of spherulites and vesicles up or down along the trend of the fissure. 
Mr. Clough has observed that in some dykes, while the amygdaloidal 
vesicles are large and undeformed in the centre, they become elongated and 
inclined downward in the direction of the margin, as if the central portions 
had not only remained fluid longer than the rest, but had a tendency to rise 
upwards in the fissure, though there was obviously less motion after these 
central vesicles appeared than in the marginal parts where the vesicles are 
so much drawn out. 
13 . BRANCHING DYKES AND VEINS 
It might have been anticipated that the uprise of such abundant masses 
of molten rock, in so many long and wide fissures, would generally be attended 
with the intrusion of the same material into lateral rents and irregular 
openings, so that each dyke would have a kind of fringe of offshoots or 
processes striking from it into the surrounding ground. It might have 
been expected also that dykes would often branch, and that the arms would 
