352 
VOLCANOES OF THE UPPER OLD RED SANDSTONE book v 
westward for a short distance, the whole volcanic zone is found to die out 
and the sandstones below and above it then come together. 
The interest of this little volcanic centre in Hoy is heightened by 
the fact that the progress of denudation has revealed some of the vents 
belonging to it. On the low ground to the east of the Cam, and imiuedi- 
ately to the north of the volcanic escarpment, the flagstones which there 
emerge from under the base of the unconformahle Tipper sandstones are 
pierced by three volcanic necks which we may with little hesitation 
recognize as marking the sites of vents from which this series of lavas and 
tufts was discharged (Fig. 105). The largest of them forms a conspicuous 
1. Caithness flagstones ; 2. Volcanic bajid lying on red suudstones and rioiigloinerates and dying out eastwards ; 3 3. 
Two vents between the base of the liillsand the sea ; their connection with the volcanic band is shown by dotted 
lines ; 4. Overlying mass of Upper Old Red Sandstone forming the hills of Hoy. 
hill about 450 feet high, the smallest is only a few yards in diameter, and 
rises from the surface of a flagstone ridge. They are filled with a coarse, 
dull-green, volcanic agglomerate, made up of fragments of the lavas with 
pieces of hardened yellow sandstone and flagstone. Around the chief vent 
the flagstones through which it has been opened have been greatly hardened 
and blistered. The most easterly vent, which has been laid hare on the 
beacli at Bring, due east of Hoy Hill, can he seen to pierce the flagstones, 
which, although their general dip is westerly at from 10° to 15°, yet at 
tlieir junction with the agglomerate are bent in towards the neck, and are 
otherwise much jumbled and disturbed. 
On the northern coast of Caithness I have described a remarkable 
volcanic vent about dOO feet in diameter, which rises through the upper- 
most group of the Caithness flagstones. It is filled with a coarse 
agglomerate consisting of a dull-greenish diabase paste crowded with 
Ijlocks of diabase, sometimes three feet in diameter, and others of red 
sandstone, flagstone, limestone, gneiss and lumps of Idack cleavahle 
augite (Fig. 106).^ The sandstones around it present the usual disrupted, 
indurated and jointed character, and are traversed by a small diabase dyke 
close to the western margin of the neck. Another similar neck has since 
been found by the officers of the Geological Survey on the same coast. 
' See Trans. Boy. Soc. Edin. xxviii. (1878), p. 405 ; also p. 482 of the same volume for au 
account of the cleavahle augite. 
