Hoclcs of Shetland. 
^01 
l3urated blue quartz. In this last substance was contained, the 
sparing quantity of copper-ore, which was the sole object of the 
mining operations. In the clay-slate of Fitful Head, at the 
Girths of Quendal, is found a vein or probably bed of iron-mica, 
noticed by Dr Fleming. It is about twelve feet in breadth, and 
appears very rich. 
I now proceed to show the relations of this system of stra« 
ta to contiguous rocks, under the impression, that the most 
important inquiry regarding Strata, is to discover under what cir- 
cumstances their superior edges or outgoings become first mani- 
fest. This inquiry involves the relations which strata exhibit 
at those lateral edges of attachment, by which they are connected 
with rocks of a different nature, and at various angles, in the 
manner that the edges of metallic plates are joined to the sides 
of a galvanic trough. There is this difference in the junction 
of a stratum, that it may be at any angle formed by its edges 
besides a right one, and at any inclination of its lateral plane, 
besides one that is vertical. This attachment of the strata, by 
their extreme or lateral edges, must of course be sought for in 
the line of their direction, and at opposite points of the com- 
pass ; as in the present instance, at the southerly and northerly 
boundaries of Fitful Head, the site of the rocks here describ- 
ed. It is unfortunate, for the sake of our first illustration, 
that the invasion of the sea should cut off altogether the con- 
nections of the southerly lateral edges of the strata ; but, tra- 
cing the tabular seams in a direction of north 15° east, we ob- 
serve the strata at their northerly extremities or lateral edges 
coming in contact with a mass of sienite, which, from the great 
quantity of epidote admitted into its composition, may be pro- 
perly named an Epidotic Sienite. The covered state of the 
ground prevents us, except in one or two places, from observing 
the strata actually in contact with the sienite. But here it is 
proper briefly to hint at the difficulty there is of determining in 
what relations strata exist with regard to an unstratified rock, 
from observations made only at the place of junction. 
An illustration to this effect is seen in a well exposed SQction of 
the junction of a few strg,ta of gneiss and mica-slate south of Noss, 
and to the N. W. of Fitful Head. Here the rocks so frequently 
pass into each other, by an interchange of substance, so rnanjT 
VOL. I. NO. S. OCTOBER 181 9o % 
