73' 
of the Rochs of Shetland. 
sion must have been essential to the primary condition of rocks, 
or to the particles which make up their ingredients. The con- 
trary, indeed, is rendered more probable, by appearances ob- 
servable in other parts of the world, which are with equal diffi- 
culty attributable to the decomposition of pre-existing rocks, or 
to debacles. 
STRATA CONNECTED WITH THE SlENITIC AND LIMESTONE MAS- 
SES OE THE CENTRAL DISTRICTS OF SHETLAND. 
The examination of the central districts of Shetland, chiefly 
comprised in the Mainland and the islands in the Bay of Scallo- 
way, will afford additional reason for regarding certain rocks in the 
light of nuclei, as affording attachments to the lateral edges of 
strata, and determining the extent and direction of their course. 
The rocks which are principally concerned in the distribution of 
the strata now to be described, are four in number. The first 
is a mass of epidotic sienite, which may be designated, from its 
situation, as the south-westerly mass of epidotic sienite ; the se- 
cond is a massof limestone extending from Weesdale Voe to Dale’s 
Voe in the parish of Belting ; the third is a mass of sienite occur- 
ring in the vicinity of Olnasfirth Voe ; and the fourth is a consi- 
derable mass of sienite, chiefly occurring in Hagrasetter Voe 
and Yell Sound. 
Now, since the order in which I notice the strata is, as they 
are crossed from east to west, their course being at the same 
time traced from their southerly to their northerly points of at- 
tachment to other rocks, the strata to be described will, in rela- 
tion to the nuclei which appear to determine their course, distri- 
bute themselves in an order that may be easily followed. Af- 
ter, 1st, describing certain strata of St Ronan’s and Colsay, 
the relations of which, from the advances of the sea, are very 
obscure, and which, on bare probability, are supposed to main- 
tain a course from the south-westerly mass of epidotic sienite, 
I shall, 2dly, proceed to describe strata with less doubtful 
relations, as probably maintaining a course from the south-west- 
erly mass of epidotic sienite, until they are observed to be inter- 
cepted very far north by the limestone of Weesdale Voe and 
Dale’s Voe : 3dly, will be described strata, the relations of 
which are sufficiently evident ; these maintain a course from the 
south-westerly mass of epidotic sienite, to the sienite of Olnas- 
