786 B. N. PEACH AND J. HORNE ON THE 
Caithness and Orkney ; and the strata in which they are imbedded, 
altered though they be, must be relegated to that period. 
This conclusion is still further strengthened by the occurrence in 
these rocks of interbedded porphyrites and tuffs ina highly altered 
form, which we detected on the headlands between Aith Ness and 
Clouster, and on the western shore south of Dales Voe, resembling 
in many respects the contemporaneous volcanic rocks to be described 
presently. Further, we are inclined to believe that the series of 
altered thick-bedded sandstones and shales which occupy the greater 
portion of this peninsular tract are on the same horizon with the 
Lerwick Sandstones on the eastern side of the Mainland. 
It is not improbable that the alteration of the strata in this wide 
area may be due to the existence of a mass of granite underneath 
these rocks. We shall have occasion to refer to the mass of granite 
in the heart of these beds in Sandsting, and to similar intrusive 
masses of Old-Red-Sandstone age to the north. The extent of ground 
occupied by these acidic rocks indicates the great volcanic activity 
which prevailed during that period; and though these are now 
isolated at the surface, it is highly probable that they may be con- 
nected underneath. 
These altered fossiliferous strata are brought into conjunction with 
the gneissose rocks to the east and north by two great faults which 
we have traced on the ground, the one running north and south, and 
the other approximately east and west. Usually the altered strata 
are terribly shattered and baked close to the lines of dislocation, 
and are likewise injected with numerous veins of very fine-grained 
felsite. 
Contemporaneous Igneous Rocks. 
In the western district of Northmavine, between Stennis and 
Ockren Head, at the mouth of Roeness Voe, there is an important 
development of ancient lavas and ashes, associated at certain loca- 
lities with ashy sandstones and red flags belonging to this period. 
The tract of ground occupied by these rocks measures about six 
miles in length, and varies in breadth from one to two miles. ‘The 
structure of this narrow tract is comparatively simple, as the strata 
form a flat syncline, in the centre of which lies coarse ash, and un- 
derneath a series of slaggy porphyrites with occasional beds of red 
ashy sandstones and flags. 
On the south bank of Roeness Voe, rather more than a mile from 
Ockren Head, in a steep grassy geo, the slagey porphyrites are 
brought into conjunction with the pink quartz-felsite by a fault. In 
Braewick Bay, west of Hillswick, the interbedded and intrusive 
igneous rocks are not found in such close proximity, the junction 
being concealed by the sandy beach; but there can be little doubt 
that the same fault runs out to sea in this bay. 
Crossing the coarse volcanic breccia, which forms the centre of the 
syncline, to Ockren Head, at the mouth of Roeness Voe, the succes- 
sive lava-flows are admirably shown, piled on each other in regular 
succession, This headland, as well as the adjacent stack, exhibit at 
