THE GEOLOGY OF THE FAROE ISLANDS. 231 
the intrusion of a sheet of dolerite. Occasionally such a sheet will leave its 
usual horizon, and, after rising through a considerable thickness of sandstone 
and shale, will push itself laterally into an upper seam, and continue along that 
line for some distance until it may ascend to a yet higher coal which it will 
“burn” in the same manner as the others. The coal-beds in the Carboniferous 
series of Scotland would thus appear to have been “lines of weakness.” In 
like manner, the coal-bearing beds of Suderde would seem to have yielded 
more readily to the assaults of the intrusive basalt than the harder and less 
easily divided anamesites with which they are associated. It is remarkable, at 
all events, that nowhere else in the island do such intrusive sheets occur. We 
could hardly have missed seeing them had they been present, for each cliff and 
mountain-slope is a magnificent geological diagram, in which every detail of 
structure is graphically exhibited. 
Dykes and veins of basalt, however, were noted in the cliffs between 
Famarasund and Faméyé. These seemed to trend N-W. by N., nearly in the 
same direction as the coast. They sent out numerous small veins and threads 
in all directions, and appeared to be of the same character as the similar dykes 
which we observed in great abundance throughout the northern islands. 
7. Contemporaneous or Bedded Dolerites of the Northern Islands.—As I 
have already indicated, the basalt-rocks of the northern islands (Stromée, 
Osterée, Sandie, &c.) are upon the whole more coarsely crystalline than those 
of Suderée, and rather dolerites than anamesites. But, just as in Suderée 
we occasionally come upon sheets of coarse dolerite interstratified with aname- 
sites, so in the northern islands anamesites are now and again encountered 
among dolerites. The prevailing colour of the dolerites of the northern islands 
is some shade of blue, but there are many black, grey, red, and purple varieties. 
All are composed essentially of augite, plagioclase, and magnetite, and most 
contain olivine, which is often abundant. Some of them are extremely coarse 
in texture—the crystals of plagioclase with which the rocks are invariably 
porphyritic, sometimes reaching half an inch or more in length,* and being often 
developed in great abundance, so much so indeed as occasionally almost to 
obscure the matrix in which they are embedded. It is this markedly porphy- 
ritic character which serves to distinguish the basalt-rock of the northern islands 
from those which in Suderde underlie and immediately overlie the coals. 
Among the most beautiful porphyritic dolerites which we saw were those of 
Skaapen in Sand6e, of Hoyviig near Thorshavn, and of the hills at Storevatn 
near Leinum. The weathering of the augite, when the crystals are distinguish- 
able, often imparts a reddish-brown tint to the rock on slightly weathered 
* FoRCHHAMMER says they sometimes reach an inch in length. This able geologist’s geognostic 
descriptions are in general singularly lucid, and he has given in small compass what seemed to us a per- 
fectly accurate account, so far as it goes, of the principal features presented by the dolerites of these islands. 
