286 
THE TERTIARY VOLCANOES 
BOOK VIII 
including a chloritic mineral, nearly colourless and singly refracting in thin 
section, and a zeolite.” 
Tracing now the tuff from the west or right side of the vent, we can follow 
it to a greater distance. No abrupt line can be detected here, any more than 
on the other side, between the agglomerate and the tuff. The latter rock 
extends under the overlying plateau of basalt, at least as far west as Portree 
Loch, a distance of fully a mile, but rapidly diminishes in thickness in that 
direction. Traces of what is probably the same tuff can be detected between 
the basalts at Ach na Hannait, more than three miles to the south (Fig. 305). 
It is thus probable that from the Portree vent fragmentary discharges took 
place over an area of several square miles. 
Above the agglomerate of this vent two lavas may be seen to start 
towards opposite directions. One of these (c), already referred to, is a dull 
prismatic basalt with a slaggy bottom, its vesicles being pulled out in the 
direction of the general bedding of the section. It descends by a twist 
or step, and then lies on the inclined surface of the tuff which dips towards 
the agglomerate and seems to pass into that rock. Further east this 
basalt increases in thickness and forms the lowest of the basalt-sheets of 
the cliff. The lava that commences on the west side of the agglomerate (if) 
is a massive jointed basalt, which, though not seen at the vent, appears 
immediately to the west of it and rapidly swells out so as to become one 
of the thickest sheets of the locality. It lies upon the rudely-bedded tuff, 
and is covered by the other basalts of the cliff. 
That these two basalts came out of this vent cannot be affirmed. If 
they did so at different times, their emission must have been followed by 
the explosion which cleared the funnel and left the central mass of agglo- 
merate there. But that some kind of saucer-shaped depression was still left 
above the site of the vent is indicated by the curious elliptical mass of rock 
(e) that lies immediately above the agglomerate, from which it is sharply 
marked off. This is one of the most puzzling rocks in the district, probably 
in large measure owing to its advanced state of decay. It is dull-red in 
colour, and decomposes into roughly parallel layers, so that at a short dis- 
tance it looks like a bedded tuff, or like some of the crumbling varieties of 
banded lavas. I could not obtain specimens fresh enough to put its nature 
and origin beyond dispute. Whatever may have been its history, this 
ferruginous rock rests in a flat basin-shaped hollow directly above the 
agglomerate of the vent. The form of this depression corresponds fairly 
well with what we may suppose to have been the final position and shape of 
the crater of the little volcano. The rock that occupies the bowl dies out 
towards the east on the face of the cliff, and the prismatic basalt (e) is then 
immediately covered by the rest of the basalt-sheets of the plateau (/). On 
the west side its precise termination is concealed by grass. But it must 
rapidly dwindle in that direction also, for not many yards away it is found 
to have disappeared, and the basalts ( cl and/) come together. 
Though the decayed state of this rock does not warrant any very con- 
fident opinion regarding its history, I am inclined to look upon it as a 
