GLACIATION OF THE SHETLAND ISLES. Wises. 
dary line so uniform as we have represented. A minute examina- 
tion of this tract convinced us that the groundwork of the area, so 
to speak, is formed of metamorphic schists, which are traversed 
in all directions by large and small veins of this rock. Both the 
diorite and the schists are intersected by innumerable veins of quartz- 
felsite which were injected at a more recent date, the whole series 
of rocks forming a complicated network. 
Again, in Dunrossness, between Quendale Bay and Loch Spiggie, 
there is a mass of intrusive rock termed by Hibbert epidotic syenite, 
which is traceable northwards through the islands of Oxna, Hildasay, 
the Sandistura rocks, the Channes, and part of Papa west of Scalloway, 
to the Mainland in Bixetter Voe and onwards to Aith Voe. This 
rock varies considerably in character throughout its course ; in some 
places it is a quartz-felsite, while in the neighbourhood of Bixetter 
and Aith Voes it is a true porphyritic granite, with large crystals 
of orthoclase. There can be no doubt that it is an intrusive mass, 
because it crosses obliquely the strike of the metamorphic rocks on 
Fitful Head and the Wart of Skewsburgh ; and it is equally clear 
that the eruption was prior to the Old-Red-Sandstone period, as the 
basement breccias of that formation rest unconformably on this rock, 
and are largely made up of angular fragments of the subjacent mass. 
A similar mass of porphyritic granite occurs in Unst on the bluff 
headland of Lambaness and on the rocky promontory north of Skaw 
Bay, which likewise bears important testimony regarding the direction 
of the ice-movement. In addition to these masses there are minor 
yeins of granite, gabbro, and serpentine, some of which are indicated 
on the map. There is one fact bearing on the age of the veins of 
serpentine on the Mainland which is worthy of note ; and that is, the 
occurrence of fragments of this rock in the basement breccias of the 
Old Red Sandstone in Dunrossness. This circumstance plainly in- 
dicates that the formation of the serpentine veins in that neighbour- 
hood preceded the formation of the breccias. 
The Old Red Sandstone. 
A glance at the map will show the various areas occupied by the 
members of this formation in Shetland. Beginning with the irregular 
areas on the east side of the Mainland, the succession may be most 
readily grasped by means of the following section (fig. 1, p. 784)*. 
Owing to a series of faults which form the boundary-line between 
the metarhorphic rocks and the Old Red Sandstone, over a great 
part of the districts of Lerwick, Quarff, Conningsburgh, and Dun- 
rossness, it so happens that different zones in this vertical section 
are brought into conjunction with the schistose rocks. The true 
base of the series, however, is exposed in the neighbourhood of 
* For previous references to the Old Red Sandstone of Shetland, see Hibbert’s 
‘Shetland Isles,’ 1822; Memoirs of Wernerian Soe. yol. i. p. 162; Quart. 
Journ. Geol. Soe. vol. ix. pp. 49, 50, also vol. xv. p.413; “The Old Red 
Sandstone of Western Europe,” by Prof. Geikie, Trans. Roy. Soc. Edin. 
yol. xxviii. p. 414; ‘The Old Red Sandstone of Shetland,’ by Dr. Gibson, 
Edinburgh, 1877. 
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