CHAP. XXII LATEST ERUPTIONS IN THE NORTH OF SCOTLAND 351 
is a zone of dull-red, fine-grained tuff, banded with seams of hard red and 
yellow sandstone. This tuff zone dies out to the eastward of the Cam. 
A few miles south of the Cain the 
volcanic zone appears again as the plat- 
form on which the picturescpie natural ^ 
obelisk of the Old Man of Hoy stands. 
Here the lava runs out as a promontory 
from the base of the cliff, and on this 
projection the Old Man has been left 
isolated from the main precipice. The ^ 
cliffs of Hoy are traversed by numerous 
small faults which have shifted the z 
volcanic zone. But on the great face 
of rock behind the Old Man there ap- 
pears to be a second volcanic zone lying ^ 
several hundred feet above that just 
described. It is probably this upper 
zone which emerges from under the 
hills a mile and a half to the south at 
Black Ness in the bay of Back wick. 
A good section is there visible, which 
is represented in Fig. 104. The ordinary 
red and yellow sandstones {a) appear 
from under the volcanic rocks at this 
locality, and stretch southwards to the 
Fig. 103. — Section of the volcanic zone iu the 
Upper Old Ked Sandstone, Cam of Hoy, 
Orkney. 
1. Caithness flagstones; 2. Dull-re<l tiiif and hands 
of sand.stone ; 3. Lava zone in tliree bands ; 4. 
Yellow and I’ed sandstone. 
most southerly headland of Hoy. 
The lowest volcanic baud in the 
section is one of red sandy well- 
bedded tuffs (&). Some of the 
layers are coarse and almost 
agglomeratic, while others are 
tine marly and sandy, with dis- 
persed bombs, blocks and lapilli 
of diabase and andesite. Hard 
ribs of a sandy tufaceoius material 
also occur. These fragmental 
deposits are immediately over- 
lain by a dark-blue rudely pris- 
matic diabase with slaggy top 
(b). It is about 150 feet thick 
at its thickest part, but rapidly 
thins away in a westerly direc- 
tion. It passes under a zone 
oi red tuff (t?) like that below, 
and above this highest member 
of the volcanic group comes the 
great overlying pile of yellow and reddish sandstone of Hoy. Followed 
Fig. 104. — Section of the volcanic zone in the Upper Old 
Red Sandstone at Black Nes.s, Rackwick, Hoy. 
