GLACIATION OF THE SHETLAND ISLES. 781 
(6) Coarse-grained micaceous and hornblendic gneiss, with asso- 
ciated limestones, bands of quartzite, talcose and micaceous 
schists. 
These subdivisions are peculiarly serviceable to the glacialist, as 
they help him to determine the different movements of the ice during 
successive phases of the ice age. The representatives of the former 
series extend from Fitful Head northwards by the Bonxie and Cliff 
Hills to Laxfirth Voe; while the members otf the gneissose series lie 
to the north-west of the area just described. They occur in the 
districts of Tingwall, Weesdale, Nesting, Lunnasting, Delting, and 
along the eastern seaboard of Northmayine. The strike of these 
metamorphic rocks is generally N. 10°-20° E. ; and though opposing 
dips are frequently met with, indicating repetitions of the strata, they 
usually dip to the north of west at high angles. Hence we have a 
gradually ascending series from the schists and clay-slates of the 
Cliff Hills to the coarse micaceous gneiss west of the vale of Tingwall, 
and the massive limestones of Whiteness and Weesdale. 
To the persistent trend of the metamorphic rocks must be ascribed 
the remarkable ridge-shaped contour of the ground in the centre of 
the Mainland. The coincidence between the trend of the strata and 
that of the parallel ridges seems to indicate a direct relationship 
between the two, the denuding agents being guided in their opera- 
tions by the relative hardness and softness of the materials exposed 
to their influence. Hence it follows that we have a series of inter- 
vening hollows running parallel with the ridges, which usually ter- 
minate seawards in long narrow voes or sea-lochs. The erosion of 
these hollows has doubtless, in some instances, been due to the 
partial removal of the bands of limestone by the chemical action of 
carbonated waters, inasmuch as the outcrop of the limestones coin- 
cides with the course of a longitudinal hollow. 
The coarse-grained gneiss of Whalsey and the Outskerries, with 
the associated limestones, is merely the prolongation of the Main- 
land series; and the same remark is applicable to the gneiss occur- 
ing in Yell. 
The structure of Unst and Fetlar is somewhat different, inasmuch 
as these isles contain well-marked zones of serpentine and gabbro, 
the distribution of which has an important bearing on the question 
of the dispersal of the stones in the Boulder-clay. In the island of 
Unst, the Vallafield ridge which flanks the western seaboard, whose 
highest elevation is about 697 feet, is mainly occupied by coarse- 
grained gneiss, dipping to the south of east at comparatively high 
angles. On the eastern slopes of the ridge the gneiss is succeeded 
by grey mica-schists and green chloritic schists, and these are over- 
lain in turn by black graphitic schists. These dark schists seem to 
form a reliable horizon with reference to the masses of serpentine 
and gabbro, as they usually crop out along the margin of the areas 
occupied by these masses and generally dip underneath them. 
Though these schistose rocks form but a narrow band from Belmont 
Bay northwards to Baliasta, they oceupy a much broader area to the 
north of the latter point, constituting, in fact, the group of hills 
Q.J.G.8. No. 140. 3G 
