CHAP. XI.I 
VENTS OF THE BASALT-PLATEAUX 
287 
deposit of much disintegrated volcanic detritus washed into the hollow of 
the old crater when it had become filled with water, and had passed into 
the condition of a inaar. The peculiarly oxidized condition of its materials 
points probably to long atmospheric exposure, and an examination of the 
surrounding parts of the district furnishes more or less distinct evidence 
that a considerable lapse of time did actually intervene between the cessation 
of the eruptions of the Portree volcano and the next great basalt-floods ol 
this part of Skye. 
That volcanic eruptions from other vents continued after the Portree 
vent had become extinct is proved by the great sheets of basalt (/) that 
overspread it, and still bury a large tract of the fragmentary material which 
it discharged. At a later time a fissure that was opened across the vent, 
allowed the uprise of a basalt dyke (//), and subsequently another injection 
of similar material took place along the same line of weakness ( h ). 
Before leaving this interesting locality we may briefly take note of the 
distribution of the ashes and stones ejected by the volcano, and the evidence 
for the relative length of the interval between the outflow of the lavas below 
and that of those above the tuff and volcanic conglomerate. These 
deposits may be traced in clear sections along the base of the cliffs for a 
mile to the west of the vent. They thin away so rapidly in that direction 
that at a distance of three-quarters of a mile they do not much exceed fifty 
feet in thickness. At Camas Ban they consist mainly of a fine, dull-green, 
granular, rudely -stratified basalt -tuff, through which occasional angular 
pieces of different lavas and rough slags are irregularly dispersed. These 
stones occur here and there in rows, suggestive of more vigorous discharges, 
the layers between the platforms of coarser detritus being occupied by fine 
tuff. Some of the ejected blocks are imbedded on end — an indication of the 
force with which they were projected so as to fall nearly a mile from the 
crater. 
The upper parts of the tuff pass upward into fine yellow, brown, and 
black clays a few feet in thickness, the darker layers being full of carbon- 
aceous streaks. On this horizon the coal of Portree was formerly mined. 
The workings, however, have long been abandoned, and, owing to the fall of 
large blocks from the basalt-cliff overhead, the entrance to the mine is almost 
completely blocked up. One wooden prop may still be seen keeping up the 
roof of the adit, which is here a slaggy basalt. 
To the east and south-east of the Portree vent, extensive landslips of the 
volcanic series and of the underlying Jurassic formations make it hardly 
possible to trace the continuation of the tuff-zone in that direction. To the 
south, however, at a distance of rather more than three miles, what is pro- 
bably the same stratigraphical horizon may be conveniently examined from 
Ach na Hannait for some way to the north of Tianavaig Bay. At the 
former locality the calcareous sandstones of the Inferior Oolite are uncon- 
formably covered by the group of rocks represented in 1 ig. 305. At the 
bottom of the volcanic series lies a sheet of nodular dolerite with a slaggy 
upper surface (a). Wrapping round the projections and filling up the 
