234 DR JAMES GEIKIE ON 
of junction between the two rocks, which is frequently very irregular. 
This discoloration is probably due to the decomposition of the augite and 
olivine, and the production.of sesquioxide of iron, and the resultant rock thus 
resembles the “palagonite” of petrologists. It seemed to me, however, that 
in some cases the tuff over which the old lava flowed had been caught up 
and commingled with the latter, as I have frequently observed to be the case 
with the porphyrites of Carboniferous and Old Red Sandstone tracts in 
Scotland, as in the Cheviots, the trappean hills of Ayrshire, the Ochils, the 
Sidlaws, and other Lowland ranges. In these Scottish areas the under portions 
of the porphyrites often contain quantities of baked sandstone and mudstone, 
which have evidently been caught up while in the condition of soft sand and 
mud, and become inter-coiled and involved with the once molten rock. Some 
of the dolerites of the islands now under review appeared to be much more 
scoriaceous above and below than others. Occasionally the slaggy under surface 
did not measure more than a foot or two, while in other cases it would reach 
as many yards. The upper surfaces were likewise often scoriaceous, but, owing 
perhaps to the comparative absence of red palagonitic matter, they were as a 
rule less conspicuous than the under surfaces. Some superficial crusts which 
I saw might readily have been mistaken for volcanic agglomerate. On the 
shore at Klaksvig, for example, a fine reddish brown vesicular dolerite is seen 
with a highly scoriaceous upper surface. This crust is made up of fragments 
chiefly vesicular, of all shapes and sizes, from mere grit up to pieces 6 inches 
and 1 foot in diameter. Some of the fragments were not unlike bombs, and 
had only the crust itself been visible it would have been difficult to distinguish 
the rock from a true volcanic breccia or agglomerate. Another appearance 
presented by the upper surface of some of the dolerites has been described by 
Sir G. Mackenzie as “not unlike coils of rope or crumpled cloth, an appearance 
which we should expect to be assumed by any viscid matter in motion.” 
The beds of dolerite vary much in thickness, and it is not easy to give any 
average. Some were less than 20 feet thick, while others exceeded 100 feet. 
But as they do not preserve the same thickness throughout, it would only be 
possible to give a reliable average after a large number of individual beds had 
been followed along their entire extent, which has not yet been done. It may 
be that the thinner beds seen in a section attain a greater thickness in some other 
part of their course, and that no single bed has a maximum thickness so incon- 
siderable as 20 feet. It is remarkable, however, for what a distance a bed will 
retain an uniform thickness. One could follow some well-marked sheets often 
along the whole course of a fiord, throughout which they seemed to retain very 
much the same thickness. As giving some notion of the number of beds, I 
may mention that in the hill-slopes and cliffs between Kollefiord and Kalbaks- 
fiord we counted some twenty sheets of dolerite, separated by lines and layers 
