276 
THE TERTIARY VOLCANOES 
BOOK VIII 
the line of contact are ranch finer in grain than the rest, and have obviously 
cooled more rapidly. The contrast between them and the ordinary dolerite 
nearer the centre, however, cannot be properly understood, except in thin 
sections under the microscope. Dr. Hatch, to whom I submitted my 
specimens, observed that, in place of the structure above described, the 
marginal parts show an absence of the ophitic grouping except in small 
isolated patches. Instead of occurring in large grains or plates enveloping 
the felspars, the augite is found in numerous small roundish grains, together 
with grains of magnetite, in equal abundance and of similar size. The 
felspars are speckled over with opaque particles ; olivine has not been 
detected. 
For miles around the vent, tire plateau-rocks are of the usual type — 
black, compact, sometimes amygdaloidal, alternating with more coarsely 
crystalline decomposing bands, the separation between different sheets 
being often marked by the ordinary red ferruginous partings. But around 
the margin of the neck, they have undergone a remarkable metamorphism. 
The portions of them which adhere to the outer wall of the neck have lost 
their distinct bedding, and have been, as it were, welded together into an 
indurated compact, black to dull-grey rock, so sliattery and jointed that 
fresh hand-specimens, three or four inches in length, are not easily obtain- 
able. Especially marked is one set of joints which, running approximately 
parallel, cause the rock to split into plates or slabs. These joints are some- 
times curved. Yet, in spite of the alteration from its normal character, 
the basalt retains in places some of its more usual external features, such, 
for instance, as its amygdaloidal structure, the amygdales consisting of calcite, 
finely acieular niesotype, and other minerals. 
Examined under the microscope, this altered basalt presents “ a con- 
fused aggregate of colourless microlites (felspar ?) and innumerable minute 
granules of magnetite, these two constituents being very unequally distri- 
buted. Sometimes the colourless portions preponderate, in other places the 
opaque granules are heaped together in black patches, which may possibly 
mark the position of fused augites.” 1 
In the zone of contact-metamorphism around some of the volcanic pipes 
in the plateaux, we see changes analogous to, but less developed than, those 
which have been superinduced on so large a scale round the great eruptive 
bosses of gabbro, granophyre, etc., that have broken up the terraced basalts 
along the west coast of Scotland. I shall accordingly return to this subject 
in connection with phenomena presented by these younger rocks (p. 386). 
ii. VENTS FILLED WITH AGGLOMEKATE 
While the necks of dolerite or basalt cannot always be satisfactorily 
discriminated from bosses which may never have established a connection 
with the surface, there is no room for any doubt in this respect in the case 
of those filled with fragmentary materials. As has been already pointed out, 
1 Notes by Dr. Hatch. 
