74 
Dr Hibbert on the Dlstrihiitim 
firth Voe : and, 4thlj, strata will be noticed, which maintain a 
course from the sienite of Olnasfirth Voe, to the sienite of Ha- 
grasetter Voe and Yell Sound. , 
1. Strata^ the relations of which are obscure^ hut ccmjectured 
to maintain a course from the south-westerly mass of Epi~ 
dotic Sienite^ until they are intercepted hy the Limestone of 
House Island and Trondra, 
Here I shall take the opportunity of first noticing the south- 
westerly mass of epidotic sienite ; and, in the next place, the 
strata of St Ronan’s and Colsay. 
South-westerly mass of Epidotic Sienite. — This mass, from 
the length of its course, which is fifteen miles, far exceeding its 
breadth, which is never greater than a mile and a half, would be 
named by geologists a Dike. Being first observed near Coleness 
in the island of Burra, it may be traced in the numerous contigu- 
ous islands and rocks, bearing the names of Papa, Oxna, Cheneys, 
the Stags, Hildazoo, the Sandistura Rocks, and Foreholm : 
from thence to the Channel of Sand Voe, Bixeter Voe, and, last- 
ly, to Aith Voe, in the parish of Aithsting. The rock contains 
much epidote, besides hornblende, and is occasionally porphy- 
ritic, forming, when polished, a beautiful ornamental stone. 
Strata (f St Ronan’^s and Colsay. — At St Ronan’s, a small 
peninsula near Dunrossness to the west of the Cliff Hills, we 
cross a series of strata, consisting of mica-slate, and of a gneiss 
which, in the skerries to the west of St Ronan’s, is porphyritic. 
The dip of the strata is to the west, at various angles, averaging 
45°. 
It may be stated, that the mass of epidotic sienite, repre- 
sented by numerous islands in the bay of Scalloway, is probably 
continued through the channel of the ocean, so as to meet the 
sienite of a similar kind in Dunrossness. For if the lines in the 
annexed map, which represent the bounding sides of each mass 
of sienite, be produced in very nearly the customary direction 
in which we have traced them, they will be found to join, or to 
be continuous. The lines also would pass so near the south of 
Colsay and St Ronan’s, that the strata would appear to derive 
their course from the epidotic sienite, until, as we may infer 
from the direction of the strata, they are intercepted by the 
limestone at House Island and Trondra to the w^est of the Cliff 
Hills. 
