GLACIATION OF THE SHETLAND ISLES. 789 
Our conclusion regarding the Roeness plateau is strengthened by 
a consideration of the relations of the granite mass of Sandsting to 
the altered Old-Red strata of that district. This siliceous intrusive 
rock has several lithological varieties ; but away from the margin of 
the area, as, for instance, on the hills above Gruting and round 
Skelda Voe, it is an ordinary granite consisting of pink orthoclase, 
quartz, and mica. 
On the shores of Gruting Voe, at the foot of Culswick Hill, the 
junction of the granite with the Old-Red quartzites and shales 
is well seen, from an examination of which it is evident that the 
granite is intruded along the lines of bedding of these strata. The 
junction-line has nearly the same inclination as that of the quartzites, 
which dip to the north at an angle of about 20°. The two rocks 
are not strictly conformable, however; for the granite here and 
there cuts across the bedding, indicating in an unmistakable manner 
the intrusive nature of the igneous rock. The junction is a sharp 
and well-defined line, as small hand-specimens can easily be 
got, 2 inches across, including the granite and the quartzite, the 
two being firmly welded together. Near the junction of the two 
rocks the quartzites are pervaded by numerous dykes of pink felstone 
proceeding from the main mass. 
The mass of granite in the north of Delting and on the western 
shores of Sulem Voe was probably erupted during the same period 
of voleanic activity ; but the evidence is not so convincing as that 
referring to the areas already described. 
But in addition to the grand series of intrusive rocks we have just 
indicated, there is evidence to prove that even these quartz-felsites 
and granites were invaded by a still later series of dykes, of a 
basic character. Hibbert detected the existence of these dykes 
on Roeness Hill; and during our traverses in the district of North- 
mayine, Delting, and Meikle Rooe we came across many similar 
masses, varying in breadth from 2 feet to several yards. Along the 
cliffs of Roeness Voe, and in the island of Meikle Rooe, these dykes 
are strikingly exhibited, forming great wall-like masses, running 
generally in a north-and-south direction. Sometimes they project 
above the acidic rocks, while, again, they weather more rapidly, 
forming great clefts in the face of the cliff. They are fine-grained, 
and consist of a dark-green diabase porphyrite. They traverse the 
metamorphic rocks, as well as the porphyrites and tuffs, west 
of Braewick; and there can be little doubt, therefore, that they 
form the last indications of volcanic activity during the Old-Red- 
Sandstone period in Shetland. 
Close by the entrance to the Noss Sound, on the Bressay shore, 
we detected a series of necks arranged in a linear manner, which 
seem to have come to the surface along a line of fissure. Similar 
necks occur on Noss, on the opposite side of the Sound. It is 
highly probable that these volcanic orifices served merely as vents 
for the discharge of steam, with occasional showers of triturated 
materials derived mainly from the sides of the vents. The adjacent 
bed of tuff, associated with the grey flags, as well as the nature of 
