232 DR JAMES GEIKIE ON 
faces. The olivine occurs either as dark or pale green rounded and amorphous 
granules or imperfect crystals, but very frequently it is more or less altered 
into serpentine. So abundant is the magnetite that it often strongly affected 
the compass, and one had to be on one’s guard while taking the direction of 
dips and glacial striz. Some of the rocks at Eide in Osterée were particularly 
magnetic. The fracture of the dolerite varies of course according to the 
character of the rock; sometimes it is smoothly conchoidal, but more generally 
it is somewhat irregular, more particularly in the case of the highly crystalline 
and more coarsely porphyritic varieties, some of which have quite a hackly 
fracture. 
I selected for microscopic examination a number of specimens which might 
be taken as fairly representative of the rocks of Suderée and of the northern 
islands. Like the anamesites, the dolerites of the northern islands differ chiefly 
in texture and the varying proportion of their component minerals. In some 
cases, as at Skaapen, the base of the rock is. quite compact like that of a basalt; 
in many others it is crypto-crystalline, like that of the anamesites of Suderde ; 
while in yet others the texture is coarse and granular, and between these 
varieties there is every gradation. Again, some of the dolerites are much more 
abundantly and coarsely porphyritic than others. Occasionally the dis- 
seminated crystals of plagioclase are small and few in number, while in some 
rocks, as already mentioned, they are large and thickly mterlaced. The 
crystals of olivine also occasionally attain a large size. Owing partly to these 
differences and partly to structural peculiarities, the dolerites are variously 
acted upon by the weather, as will be afterwards pointed out more particularly. 
I found that as a rule the more highly amygdaloidal portions of a rock suc- 
cumbed most readily to the denuding forces—the denser or less amygdaloidal 
areas often projecting beyond these latter for several feet. 
Amygdaloidal and non-amygdaloidal areas frequently alternate in more or 
less regular bands which are parallel to the plane of bedding, and several of 
these alternations might be observed in one and the same bed, the line of 
separation between the bands appearing at the distance of a few yards to be 
often well-defined (see Plate XIV. fig. 11). The matrix of the amygdaloidal areas 
was frequently finer grained than the other parts of the rock ; it varied also in 
colour, and was often dull and earthy, becoming soft, wacké-like, and crumbling. 
The non-amygdaloidal bands, on the other hand, were generally coarser grained, 
crystalline, and hard. The more crystalline parts of a dolerite might thus be 
black or blue, while the amygdaloidal portions were pale red or brown, grey, 
yellow, or dirty green. Small amygdaloidal cavities often occurred more or 
less numerously in the harder crystalline bands, along the line of junction with 
the wacké-like areas; but, so far as my observations went, they quickly dis-— 
appeared at the distance of a few feet from the junction-line, although a few 
