235 
of the Rocks of Shetland. 
tions, the drusy cavities of which are lined with quartz-crystals. 
North-east of the bank of Ollas Voe, the cl ay stone-porphyry is 
occasionally prone to the conglomerate structure, and we find it 
alternated with thin beds of sandstone. 
West of Ollas Voe, the porphyry presents no remarkable ap- 
pearance ; but near the most northerly point of the headland 
between Ollas Voe and Culia Voe, conglomerate rocks rest on 
the porphyry. East of Culia Voe, whilst claystone-porphyry is 
the lowest rock, conglomerate strata, into which the porphyry 
passes, are the next in point of superimposition, and an amyg. 
daloidai rock is the highest in the series. Here the porphyry 
is imperfectly stratified. West of Culia Voe, the claystone-por- 
phyry is uncombined with any other rock. 
North-west of the Island of Papa Stour, near the entrance in- 
to Culia Voe, conglomerate strata rest, in nearly a horizontal po- 
sition, on claystone-porphyry. But a little to the westward, we 
find the porphyry in a reversed relation, appearing to rest upon 
a conglomerated sandstone, and, to the south-west, upon a rock 
consisting of alternations of conglomerate and amygdaloidal stra- 
ta. Here it is very probable, that the apparent superimposition 
of the porphyry merely involves the contact of one portion 
only of its bounding surface, which is impendent ; for, as we 
pursue our course to th6 south-west, we find that a sandstone, 
which is connected with all the strata apparently subjacent, now 
comes in contact with another portion of the bounding surface of 
the porphyritic mass, which is inclined. Consequently, the 
same strata which in one place appear to be subjacent to the 
porphyry, are now the superimposed masses. From all these 
anomalous circumstances, it seems very evident, that the terms 
Superimposition and Subjacency, may lead to very erroneous no- 
tions in regard to the attachments of strata ; since they involve 
contradictory appearances, which can only be reconciled, on a 
supposition that the edges of strata may be variously opposed 
to two different kinds of surfaces, described as impendent or 
inclined^ and that these, at every step we take, may diversify 
the form of the individual rock affording attachment to strata. 
Quitting these conglomerate and amygdaloidal strata of the 
north-west of Papa. Stour, our examination 6f the island con- 
cludes with the uninterrupted continuation of the claystone- 
porphyry to the west and south-west of the island, until we 
Q 2 
