78 Dr Hibbert on the Distribution 
scribed. The dip is generally to the west, at angles very 
nearly vertical. 
Gneiss extending from Foraness and Rusaness to Grohus- 
ness in Olnasfirth Voe. — The gneiss thus described, which is 
much traversed by insulated veins of granite, has a direction of 
N. 12“ E., dipping to the west at angles of about 45°. The 
course of the strata is from the epidotic sienite of Sand Voe 
and Bigsetter Voe to the sienite of Olnasfirth Voe. In the 
gneiss of Selaness in Aithvoe, may be observed the vein of gra- 
nite to which I formerly alluded in speaking of the conglomerate 
rocks. It contains rounded concretions six or eight inches in 
length, and these, if we were ignorant that they were taken 
from a rock exhibiting in no other instance appearances of a 
mechanical origin, would be readily conceived of as water-Worn 
pebbles. The vein is six feet in width. 
At the Burn of Taitagill, where the gneiss and the Weesdale 
mica^slate pass into each other, an extraordinary quantity of por- 
celmn earth is accumulated, by the decomposition of the felspar 
which the gneiss contains. Na experiments have yet been 
made to ascertain if the substance is applicable to the arts : 
it is thus found on the estate of John Ross, Esq. of Weesdale. 
4. Strata maintaining a course from the Sienite of Olnas- 
firth Voe to the Sienite of Hagrasetter Voe and Yell Sound, 
Our notice of these strata, previously leads us to that of the 
most northerly mass of sienite remaining to be described. 
The Sienite of Hagrasetter Voe and Yell Sound. This mass 
of sienite presents, in a horizontal section, somewhat of a pyri- 
form appearance, the outline of which may be traced by com- 
mencing our investigation from the grounds of Thomas Gifford, 
Esq., close to whose mansion the rock first appears. The 
boundary line may then be traced north, along the western 
banks of Hagrasetter Voe, the western side of the peninsula of 
Glus, the extremity of Ollaberry Ness ; then in a south-easterly 
direction in Yell Sound, through the islands contiguous to each 
other, named Brother Island, Little Rhoo, Riga, and Samph- 
ray ; thence to Firth’s Voe in the Mainland, and in a westerly 
direction to Girthness Voe. It is then continued in the bed of 
Hagrasetter Voe, re-appearing, however, in the isthmus divid- 
2 
