GLACIATION OF THE SHETLAND ISLES. 787 
least four different lava-flows, capped by coarse ash. The lavas 
thicken and thin out rapidly, and likewise exhibit. the usual slaggy 
structure at the top and bottom of the flow. Some of these beds 
are also highly involved, and show clearly the way in which the 
partially solidified crust has been caught up and rolled over and over 
in the advancing current of still molten lava. 
We have already alluded to the porphyrites and tuffs which occur 
in the altered rocks north of Walls. We also detected a bed of lava 
in the Holm of Melby, and a thin bed of tuff associated with the 
grey flags on the east side of Bressay, opposite the north end of the 
island of Noss. The contemporaneous volcanic rocks found in Papa 
Stour have been previously described by Professor Geikie. 
Intrusive Igneous Rocks. 
In the north and western portions of the Mainland there is a 
splendid development of highly siliceous intrusive rocks, which 
occupy the most elevated ground in the island. They extend from 
a point on the north end of the Mainland opposite the island of Uya, 
southwards to Roeness Voe, culminating in the dome-shaped mass 
of Roeness Hill. Thence they cross the peninsular tract to the 
Heads of Grocken, west of Hillswick, reappearing in the slender 
columns of the Drongs. The western portion of Meikle Rooe is formed 
of the same material, and likewise the north-eastern headlands of 
Vementry, while the small area of quartz-porphyry at Melby must 
also be included in the same great intrusive series. In addition to 
the areas now referred to, there are other lenticular masses varying 
in size down to veins a few feet across, occurring at intervals from 
Mavis Grind, northwards to Roeness Yoe and Olaberry. These tra- 
verse the Northmavine diorite and metamorphic rocks alike, in- 
creasing in number and extent as they approach the Roeness mass. 
These rocks vary considerably in character ; but they all agree in 
possessing a large proportion of silica, while the felspar is usually 
orthoclase. As a rule, they are coarsely crystalline, the two pre- 
valent ingredients, quartz and orthoclase felspar, being distinctly 
crystallized, which causes the rock to assume a marked granitoid 
texture. There can be little doubt that these coarsely crystalline 
rocks must have originally consolidated under great pressure, though 
the materials under which they lay buried have been wholly re- 
moved by denudation. Further, the marked columnar structure 
which meets the eye along both banks of Roeness Voe, and from the 
Heads of Grocken to Braewick Bay, as well as along the western 
shores of Meikle Rooe, suggest the idea of a great intrusive sheet, 
forced in like a wedge between the metamorphic series and the 
members of the Old Red Sandstone long since worn away. A simi- 
lar intrusive sheet occurs in Papa Stour, as described by Professor 
Geikie, where the same columnar structure is apparent, and where 
a fragment of the once superincumbent strata is still to be seen at 
the Horn of Papa. Fortunately the intrusive nature of this latter 
sheet is placed beyond doubt, inasmuch as the pink porphyry is seen 
