71 
of the Rocks of Shetland. 
dependent form, which, in relation to the strata attached to 
them, I have conceived of in the light of nuclei, since they 
assume every variety of position and form, and meet strata 
at all possible angles. Thus I consider, that a study of the 
conditions of junction, under which small insulated masses of 
granite, sienite, porphyry, limestone or hornblende, are found 
with the strata which they traverse, will afford the most direct 
examples of the distribution of rocks of a similar kind, when, 
from their greater magnitude, we may consider them in the 
light of mountain masses. The rocks which disturb the course 
of strata, appear in fact, independently of magnitude, to differ 
from ^each other in two respects only. Certain strata attached 
by their lateral edges to rocks which I have regarded under the 
character -of nuclei, may, in the first place, be observed to ter- 
minate altogether at the place of junction. From no other re- 
maining points of the mass is a resumption of such strata to be 
found. On the contrary, either a new series or system of rocks, 
or strata of a similar kind, arranged in a totally different order, 
or perhaps with a direction and inclination equally varying, de- 
rive their origin from the remaining and generally opposite 
points of the nucleus. Secondly, Certain strata attached to a 
rock by their lateral edges, are not altogether terminated or in- 
tercepted, but only by such an occurrence interrupted in their 
course. W e find that, from other points of the interrupting mass, 
similar strata are resumed, but frequently with some disturb- 
ance in their line of direction or dip. Under this view, every 
small insulated mass or vein which traverses strata, is a nucleus 
of a subordinate kind, which interrupts and disturbs, but does 
not eventually terminate the course of strata. 
the sthata of lunna ness, 
Lunna Ness is a considerable headland, constituting the north- 
easterly extremity of tbe Mainland of Shetland. 
These strata consist of gneiss, appearing, from their proximity 
to the rocks of the same name last described, to form a part 
of them. The strata, however, of Lunna Ness are a distinct 
system. They exhibit certain imperfect manifestations of a con- 
glomerated structure, by containing angular portions of quartz, 
which, however, most frequently graduate into the substance of 
the containing rock. The strata are traversed by numerous im. 
sulated veins of granite, and occasionally by those of sienite. 
