3i« 
THE TERTIARY VOLCANOES 
BOOK VIII 
Reference may be made, in conclusion, to a not infrequent feature of the 
Skye sills. Like the dykes, they are often double or multiple, molten 
material having been successively injected along the same plane. The 
example just cited from the west side of Sleat illustrates one type of such 
compound sills. More frequently, however, the subsequent injections have 
been made along the floor or roof of the first sheet. Mr. Marker has found 
numerous cases of this structure in the Strath district. They are recogniz- 
able even from a distance by their terraced contours when seen in profile. 
1 hey often vary considerably in thickness owing to the dying out or coming- 
in of their separate bands ; while, on the other hand, single sills tend to 
maintain a uniform thickness for long distances, or taper away gradually. 
The compound arrangement of the basic sills is well brought out where acid 
material has been injected between the sheets, as will be more fully described 
in Chapter xlviii. 
111. EIGG, ARDNAMUKCHAN 
Mull. 
Eigg. 
The phenomena of the coasts of Skye are repeated on the east side of 
in Eigg, and still more magnificently along the south coast of 
A single example is here given (Fig. 325) from the east side of 
Over the Jurassic sandstones ( a a) a sill of basalt (1) four to six feet 
thick has been injected between the 
stratification, and another (2) two 
to four feet thick has forced its way 
across the middle of one of the bedded 
basalts (6 b) in which it bifurcates, 
and above which comes the thick 
series of lavas of the plateau (c, cl). 
In one of the streamlets, which 
exposes a section of the Jurassic 
strata below the volcanic escarp- 
ment, more than twenty intrusive 
sheets may be counted among the 
shales and limestones. They are 
sometimes not six inches thick, and 
seldom exceed six or eight feet. 
I will conclude this account of 
the Tertiary basic sills of Britain 
by referring to one further district 
in the West of Scotland, where 
they are well displayed on bare 
hill-slopes and also along a picturesque sea-coast. In the promontory 
of Ardnamurchan in the west of Argyleshire, one of the most con- 
spicuous eminences, known as Ben Hiant, affords a striking mass of intrusive 
material, which, extending along a rugged shore for three-quarters of a mile, 
mounts thence inland in a series of rocky knolls, and in rather less than a 
-A*,— 
Pig. 325. — Section to show Bedded and Intrusive 
Sheets, Eigg. 
