378 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
( ) Dolekites and Basalts^ — These rocks are found both as interstrati- 
ed lavas and as intrusive masses. In the former condition they take a 
conspicuous place among the sheets of the plateaux, but especially in the 
lower parts of the series. They are dark, often black, usually more or less 
porphyritic, with large felspars, frequently also large crystals of augite or 
ohvme, and may be described as porphyritic olivine-dolerites and olivine- 
basalts, more rarely as olivine-free dolerites and basalts. Their groundmass 
consists of short laths or microlites of felspar (probably labradorite) and 
granules or small crystals of augite and magnetite, with sometimes a little 
fibrous brown mica. The large porphyritic felspars are striped (probably 
labradorite), the augites are frequently chloritized, and the olivines are 
generally more or less serpentiiiized. But in some cases all these minerals 
are as fresli as m a recent basalt. The rocks are sometimes beautifully 
columnar, as at Arthur Seat. 
Of these basic lavas conspicuous examples may be seen at Arthur Seat 
Calton Hill^and Craiglockhart Hill. The eastern part of Arthur Seat,’ 
known as Whinny Hill, furnishes examples of olivine-dolerites of the Jed- 
burgh type (p. 418). The beautiful basalt of Craiglockhart with its large 
porphyritic olivines and augites has afforded a distinct type of Carboniferous 
basalt (p. 418). The same type occurs on the Calton Hill in the cliff 
below the gaol. Similar basic lavas are especially abundant and remarkable 
111 ilie Clyde plateau near Campbeltown in Argyllshire, and at the south end 
of Bute and in the Cunibraes, where they are associated with an interesting 
senes of dykes and sills. But even where, as in the Garleton Hills, the lavas 
are for the most part somewhat acid in composition, those first poured out, 
which fonn the lowest band, include some typical olivine-basalts, of which a 
characteristic example occurs at Kippie Law at the base of the Garleton 
plateau (p. 418). It has been described by Dr. Hatch as exhibiting under 
the microscope porpliyritic crystals of felspar and olivine lying in a ^round- 
mass composed of lath-shaped felspars, granular olivine and magnetite, and 
microlitic augite.^ The olivine, originally the most abundant constituent, has 
been converted into^ a fibrous aggregate of serpentine. All the minerals 
are more or less idiomorphic, hut especially the augite, which crowds the 
poundmass in delicately-shaped prisms, most of which are terminated at 
both ends by faces of the henii-pyramid. The analysis of this rock is mven 
in the accompanying table of analyses of Garleton basalts. ITie Ivippie'’Law 
type of basalt was recognized by Dr. Hatch among the Geological Survey 
collections from other districts, as in the intrusive bosses of Xeides Law 
and Bonchester near Jedburgh, and from the Campsie plateau a mile and a 
half north of Leniio.xtown." 
At Hailes Castle, in the Garleton plateau, the lower basic lavas include 
another olivine-ba.salt somewhat more felspathic than that just described, and 
“ A general cla.s.sificat.ioii. of the whole series of Seottish Carboniferous dolerites and basalts 
irChipte XX\ L £ “ 8 )“’ given in the account of the rocks of the pny.^ 
Trans. Roy. Soc. Edin. vol. .x.\.\'vii. ( 189 . 3 ), jip. 117 - 119 . 
