460 PROFESSOR GEIKIE ON THE 
hand, which were discharged by the eriform explosions from the cool upper 
crust, on the first outburst of a vent, would not exhibit any trace of such a 
change. Where, therefore, we meet with a neck full of fragments of unaltered 
stratified rocks, we may suppose it to have been that of a short-lived volcano ; 
where, on the other hand, the fragments are few and much altered, we may 
infer that they mark the site of a vent which continued longer active. 
2. The volcanic rocks of the necks consist occasionally of (a) some form of 
lava, but more usually of (4) fragmentary materials, with or without veins and 
pipes of lava. 
(z) In various parts of the Basin of the Firth of Forth occur circular or 
oval bosses of basalt, dolerite, or porphyrite, which exactly resemble in contour 
the typical necks of tuff, and occur among interstratified rocks in such a manner 
as to suggest that they mark the sites of volcanic vents. Traprain Law and 
North Berwick Law are conspicuous examples. Each of these eminences rises 
in the midst of tuffs and lavas, and may not improbably be a portion of the lava 
column which rose in a volcanic pipe. A smaller but very perfect example 
Fig. 6.—Section of Porphyrite neck, in sandstones, Shore of Haddingtonshire. 
oF 
(fig. 6) occurs on the shore to the east of North Berwick Law.* The Castle — 
rock of Edinburgh may be another. In these cases we probably see a deeper 
part of the pipe than that in which fragmentary materials accumulated. 
(>) In the great majority of cases the necks are filled with fragmentary 
volcanic detritus. Sometimes this material consists of a coarse utterly 
unstratified mass or agglomerate of different lava blocks, angular and sub- 
angular, varying in size up to a diameter of a yard or more. The later 
agglomerate of Arthur Seat may be taken as an illustration of the coarsest 
variety. In other cases it is a breccia of small angular and subangular lava 
fragments. Most frequently it is a more or less compact or gravelly tuff, com- 
posed of a fine comminuted paste of volcanic dust and sand, full of rounded and 
subangular blocks and bombs of basalt, porphyrite, or other form of lava. In 
the east of Fife some of the necks contain a remarkable compact volcani¢ 
sandstone, composed of the usual detritus, but weathering into spheroidal crusts 
so as externally to be readily mistaken for some form of basalt rock. There 
can be little doubt that this variety of rock was originally a volcanic mud. 
The lithological details of the tuffs, however, will be given ina later part of this 
memoir. q 
* See “ Geology of East Lothian,” Geological Survey Memoir, p. 40. 
