252 
THE TERTIARY VOLCANOES 
BOOK VIII 
abundant and beautiful zeolites, the hollows of the upper surface of the 
sheet being filled in with dark brown carbonaceous shale, forming a layer 
from one to fourteen inches thick, marked by coaly streaks and lenticles (6). 
A band of green and yellow sandstone (c) next supervenes, which, from its 
pale colour, attracts attention from a distance, and led me, while yachting 
along the coast, to laud at the locality in the 
hope that it might prove to be a plant-bearing 
limestone. This sandy stratum is only some 
three or four inches thick at the north end of 
the section, but increases rapidly southward 
to a thickness of as many feet or more, when, 
owing to the cessation of the underlying shale, 
it comes to lie directly on the amygdaloid and 
to enclose slaggy portions of that rock. Im- 
mediately above the sandstone two or three feet 
of fissile shale, black with plant-remains (d), 
Fig. 285.— Intercalated group 0 f include brown layers that yield to the knife 
Sh^rn^eof^-ef 8 ' iike ^ oil ' shales - The next stratum is a 
seam of coal (e) about a foot thick, of remark- 
able purity. It is glossy, hard, and cubical, including layers that break 
iike jet. It has been succeeded by a deposit of green sand (/), but while 
this material was in course of deposition another outpouring of lava (<7) took 
place, whereby the terrestrial pool or hollow of the lava-field, in which the 
group of sedimentary materials accumulated, was filled up and buried. 
This lava is about 20 feet thick, and consists of a coarsely-crystalline, 
jointed dolerite with highly amygdaloidal upper and under surface. Its 
slaggy bottom has caught up or pushed aside the layer of green sand, so as 
to lie directly on the coal, and has there been converted into the earthy 
modification so familiar under the name of “ white trap ” among our coal- 
fields. It is interesting to find that this kind of alteration, where molten 
rock comes in contact with carbonaceous materials, is not confined to 
subterranean sills, but may show itself in lavas that have flowed over a 
terrestrial surface. 
From the frequent intercalation of such local deposits of sedimentary 
material between the basalts, we may reasonably infer that during older 
tertiary time the rainfall in North-Western Europe was copious enough 
to supply many little lakes and streams of water. As the surface of the 
lava-fields decayed into soil, vegetation spread over it, so that, perhaps for 
long intervals, some tracts remained green and forest-clad. But volcanic 
action still continued to show itself, now from one vent, now from another. 
These wooded tracts were buried under overflows of lava, and, the water- 
courses being filled up, their streams were driven into new channels, and 
other pools and lakes were formed. 
In no part of the Tertiary volcanic area of Britain can the characters of 
the lavas and the structure of the plateaux be better seen than along the 
west side of Skye, north of Loch Bracadale. The precipices rise sheer out 
