446 PROFESSOR GEIKIE ON THE 
destruction of these lavas. In the Edinburgh district, the first lavas erupted 
were anamesites and basalts, and the tuffs were formed of their debris. In 
the latter half of the volcanic period the lavas became “ porphyrites.” 
It is deserving of notice that the volcanic mass of Arthur Seat lies in the 
line of the older volcanic ridge of the Pentland Hills, and at a distance of 
scarcely two miles from the great vent of the Braid Hills. The long interval 
which separated these Lower Carboniferous volcanoes from those of the Lower 
Old Red Sandstone still left a weak part near the ancient vent. Through that 
line of weakness the volcano of Arthur Seat broke out. At a subsequent 
time, perhaps in the Permian period, another volcanic orifice was opened near, 
but not quite upon, the same site. From this last opening the upper and 
newer rocks of Arthur Seat were ejected.* 
Owing to the fact that the line of junction between the red sandstones and 
the overlying upper group of the Calciferous Sandstones is almost everywhere 
obscured by faults, it is difficult to determine the number of distinct volcanoes 
in the Edinburgh district. Arthur Seat and Calton Hill no doubt form parts 
of the ejectamenta of the same vent. I formerly suggested that this vent 
may be represented by the neck of basalt forming the Castle Rock of Edin- 
burgh. But there may have been another orifice further east, somewhere on 
the south side of Arthur Seat. I am now disposed to regard the tuff and 
anamesite of Craiglockhart Hill as the products of a separate vent which lay 
in the near neighbourhood of that locality, probably a little to the west. 
Far to the south-west, on the borders of Lanarkshire, an isolated volcanic 
cone poured forth basaltic sheets and slight showers of tuff which now form a 
band, running for several miles, as a boundary between the two groups of the 
Calciferous Sandstones. 
In Plate X. a series of vertical sections is given to show the nature and ~ 
position of the interbedded volcanic sheets in the Lothians and Fife. From 
these sections it will be observed that in the Edinburgh district, where — 
there is a maximum depth of about 500 feet of volcanic rocks, these lie near 
the base of the Carboniferous series. They occur at Arthur Seat, where the 
first eruption produced a stream of lava (Long Row), followed after an 
interval by greenish tuffs and volcanic breccias. Beautifully columnar as 
wellas amorphous basalts overlie those fragmental strata, followed by sheets of — 
dark dull-red “ porphyrite,” which form the remainder of the volcanic series. 
In the adjacent Calton Hill, the porphyrite beds are more split up with layers 
of tuff and breccia. (See fig. 24.) 
Allusion must be made here to the intrusive sheets and veins which occur 
* Descriptions of the geological structure of Arthur Seat will be found in Maclaren’s “ Geology 
of Fife and the Lothians,” and in the “Geological Survey Memoir of Sheet 32, Scotland.” Mr Jupp 
has offered an explanation of one part of the history of the hill (Quart. Journ. Geol. Soc., vol. Xxxi. p. 
131), which I believe to be quite untenable. It will be referred to elsewhere, 
te) 
