THE GEOLOGY OF THE FZROE ISLANDS. 233 
might now and again be detected through the body of the rock; and even 
occasionally a sporadic area, more or less vesicular and honeycombed, might 
be seen completely enclosed in crystalline non-amygdaloidal rock. Such 
alternating and variously coloured layers, although the individual zones are 
frequently of very irregular thickness, have yet all the appearance of true 
bedding when viewed from a little distance. 
The amygdaloidal cavities vary in size from mere points up to hollows more 
than 2 feet in diameter. Many of them are round, others are more or less 
flattened and drawn out in the plane of bedding, while yet others are quite 
irregular, and seem to have been formed by the confluence of several vesicles. 
In some places the cavities are very abundant—the rocks being literally honey- 
combed with them. When such is the case they are generally small—the larger 
cavities being perhaps most common when the rock is least porous. Some of 
the largest we saw were on the east coast of Skaalefiord. Very frequently the 
cavities occur in more or less continuous lines or layers, parallel to the plane 
of bedding (see Plate XIV. fig. 12), a feature which may be noted again and 
again in the sea-coast sections, particularly along the north-west shores of 
Stromée. Now and again also may be observed that involved appearance of 
the cavities which has already been described as occasionally visible in the 
anamesites of Suderde. The prevailing amygdaloidal minerals are chabasite, 
stilbite, mesotype, apophyllite, analcime, quartz, chalcedony, calcspar, and green 
earth. It is not uncommon to find two, three, or even four different zeolites 
in one and the same drusy cavity. 
I have said that more highly amygdaloidal parts often alternate with 
harder non-amygdaloidal zones in one and the same bed. ‘This, however, is 
far from being always the case. Sometimes the vesicular areas appear to be 
as durable as the other portions of a rock, and do not differ from these either 
in colour or texture. Frequently the dolerites seem to be tolerably homogeneous 
throughout—there being no trace of that alternation of zones just referred to. 
The under and upper surfaces, however, wherever they came under my observa- 
tion, were always more or less vesicular, and often highly slaggy and scoriaceous. 
These scoriaceous portions are very striking in appearance. They appear to be 
made up of jumbled and broken fragments of highly vesicular dolerite—varying 
in diameter from a few inches up to several feet, enclosed in a less vesicular 
matrix of the same material. At other times the matrix appears to be 
amorphous, earthy-like, and highly discoloured, generally becoming bright red, 
and showing occasional yellow and parti-coloured areas. These red discoloured 
areas so closely resemble the tuffs upon which the dolerites repose that it 
is sometimes hard to say where dolerite ends and true tuff begins. When 
the rock is highly porphyritic, however, the presence of the large crystals of 
plagioclase in the reddened portions usually enables one to detect the line 
VOL. XXX. PART I. 2N 
