CHAP. XXXIV 
PETROGRAPHY OF THE DYKES 
129 
margin of the dyke in a band about two inches broad, but nearly absent 
from the rest of the rock. Elsewhere the crystals are grouped rather in 
patches than in bands. Among the dykes south of Oban some similar 
instances of coarsely porphyritic felspars may be observed. 
Not only are these porphyritic felspars apt to occur in bands parallel 
with the outer margins of the dykes, but they tend to range themselves 
with their longer axis in the same direction, thus even on a large scale, 
visible at some distance, showing the flow -structure, which is so often 
erroneously regarded as essentially a microscopic arrangement, and as 
specially characteristic of superficial lava-streams. 
Mr. Harker in his survey of Strath, Skye, has met with some remark- 
able examples of the enclosure and incorporation of foreign materials in the 
younger group of dykes which in that district traverse the granopliyres 
and gabbros. He remarks that the great majority of these dykes are basic, 
and he has found them to be capable of convenient division into two groups. 
1st, Non-porphyritic basic dykes with a specific gravity between 2’87 and 
2‘97, and an amygdaloidal structure affording clear indication of flowing 
movement, either at the sides or along a central band. These dykes do not 
greatly differ from those of pre-granophyre eruption. 2nd, Porphyritic basic 
dykes which present features of peculiar interest. The porphyritic (or 
pseudo-porphyritie) elements, according to Mr. Harker’s observations, are 
constantly felspar, frequently subordinate augite, and exceptionally quartz. 
The felspars have for the most part rounded outlines with a bordering zone 
of glass cavities apparently of secondary origin. The augite, in rounded 
composite crystal-grains, differs from that of the groundmass and resembles 
the augite of the gabbros. The quartz-grains are likewise rounded, and 
show sometimes a distinct corroded border. 
These characters, Mr. Harker observes, are those of crystals derived from 
some foreign source, and it can scarcely be doubted that this is the explana- 
tion of their presence. He noticed that the dykes in question frequently 
enclose fragments, varying up to several inches in diameter, of gabbro, 
granite or granophyre, bedded lava, quartzite, etc., which show clear evidence 
of having been rounded and corroded by an enveloping magma, and recog- 
nizable crystals from some of the fragments may be observed in the 
surrounding parts of the matrix of the dykes. Most of the felspar and 
augite crystals disseminated through these porphyritic basic dykes may be 
referred to the partial reabsorption of enclosed fragments of gabbro. The 
same observer has found that many of the dykes which rise through the 
basalt-plateau of Strathaird are crowded with gabbro fragments. 
Another megascopic character of the material composing the dykes is 
the frequent presence of amygdales. It has sometimes been supposed that 
amygdaloidal structure may be relied upon as a test to distinguish a mass of 
molten rock which has reached the surface from one which has consolidated 
under considerable pressure below ground. That this supposition, however, 
18 erroneous is demonstrated by hundreds of dykes in the great system 
which I am now describing. But the amygdales of a dyke offer certain 
VOL. II ( K 
