The Biurgs. 
Fig. 2.—Section across Northmavine from Ockren Head to Skea Ness. 
B, N. PEACH AND J. HORNE ON THE 
cutting across the underlying sandstones 
y from a lower to a higher horizon. 
As the result of careful mapping of 
IN the boundaries of the Northmavine 
IY = mass, we are of opinion that the Roe- 
ness-hill plateau is a great intrusive 
sheet which forced its way upwards and 
laterally between the metamorphic strata 
on the one hand, and the members of the 
Old Red Sandstone on the other, at the 
time when the Mainland lay buried under 
the sedimentary deposits which accumu- 
RS lated during that period. It is not at all 
he im improbable that this immense mass may 
6 have been connected with the surface 
a \ 8 by pipes which traversed the superin- 
‘eB cumbent strata, and may have discharged 
val volcanic materials at the surface. 
ey g Its relations to the metamorphic series 
<3 are admirably defined. Along the east- 
as S, ern seaboard of Northmavine it forms a 
\ §$ mural escarpment about 200 feet high, 
r~  as.3 part of which is known by the name of 
<= the Biurgs. Innumerable veins of 
AN quartz-felsite branch off from the main 
mass and intersect the metamorphic 
series. Further, it sometimes happens 
that portions of the adjacent rocks are 
enclosed in the quartz-felsite, as, for in- 
stance, near Colifirth Voe, where a frag- 
ment of serpentine is caught up in the 
mass. Again, on the north bank of 
Roeness Voe, the sheet spreads over the 
edges of the diorite and metamorphic rocks 
without producing any deflection of the 
strike of the metamorphic series, as shown 
in the accompanying section (fig. 2). 
To the west of Hillswick, on the pic- 
turesque Heads of Grocken, the highly 
siliceous quartz-felsite is thrown against 
the metamorphic series by a fault, which 
is well seen on the cliffs. This fault 
passes out to sea between the little islets 
of Waterhouse Holm and the Drongs, 
and reappears in Meikle Rooe, separating 
the quartz-felsite from the diorite of that 
island. In all likelihood this disloca- 
tion is the northward prolongation of the 
eke great north-and-south fault already de- 
Sameer scribed, which brings the metamorphosed 
Lit Old-Red-Sandstone rocks into conjunc- 
| tion with the gneissose rocks of Weesdale. 
2. Pink granite and quartz-felsite, 
1. Metamorphic rocks. 
