782 B. N. PEACH AND J. HORNE ON THE 
round Saxavord. They reappear again in the south-east corner of 
the island, where they cover a strip of ground about a mile in breadth 
between Skuda Sound and the ruins of Muness Castle. 
The masses of serpentine and gabbro in Unst lie in a trough 
formed by these schists. They may be said to form two parallel 
zones crossing the island from south-west to north-east, the serpen- 
tine lying to the west of the gabbro. The serpentine area is the 
larger of the two, though somewhat irregular in outline; at the 
northern limit between Baliasta Kirk and Swena Ness, the mass 
is nearly two miles in breadth, but as it is traced southwards it 
diminishes to half a mile in breadth. Another patch of gabbro is to 
be met with on the promontory east of the ruin of Muness Castle. 
It seemed to us that the serpentine has resulted from the meta- 
morphism of the gabbro. Here and there in the gabbro area, as, 
for instance, on the west side of Uya Sound, lenticular patches of 
serpentine occur, as if the transmutation had partly begun and had 
been interrupted. The gradual transition from the one rock to the 
other is well seen in the promontory on the south side of Balta 
Sound. Professor Heddle, who advocates this view, states that the 
gradual passage can be seen in hand specimens on Swena Ness. 
The structure of the northern portion of Fetlar is comparatively 
simple. ‘The central hollow coincides with a low anticlinal axis of 
black graphitic schists and chloritic schists similar to those in Unst, 
and apparently occupying the same horizon with reference to the 
gabbro and serpentine. ‘hese rocks throw off on both sides of the 
arch beds of gabbro and serpentine, forming the elevated ground 
round the Vord Hill on the east and the hills near Urie on the west. 
At Urie the serpentine which overlies the gabbro is immediately 
succeeded to the west by coarse-grained gneiss, the perfectly con- 
formable junction between the two being distinctly visible on the 
shore west of the promontory of Urie. ‘The broad mass of serpen- 
tine which stretches from the Vord Hill eastwards to Gruting Bay is 
thrown into a synclinal trough, which is nowhere deep enough to 
bring in the overlying gneiss to the west of Urie.. To the east of 
Gruting Bay occur the micaceous and chloritic schists as well as the 
graphitic schists, which contain in minor folds small patches of ser- 
pentine. 
Intrusive Igneous Rocks in the Metamorphic Serves. 
Under this heading we shall only indicate those intrusive rocks 
and those products of extreme metamorphism which are probably 
older than the Old-Red-Sandstone period. We have already referred 
to the areas of gabbro and serpentine in Unst and Fetlar; but in 
addition to these there are certain masses on the Mainland deserving 
special notice. 
Of these by far the largest is the mass of diorite occurring in the 
districts of Delting and Northmayine on the Mainland. It is up- 
wards of ten miles in length, and in places it exceeds two miles in 
breadth ; but it ought to be borne in mind that the whole of the 
area now described is not occupied by the diorite, nor is the boun- 
