508 PROFESSOR GEIKIE ON THE 
sometimes distinctly fibrous, likewise occur. These various decomposition 
products give between crossed Nicols sometimes the reaction of serpentine, 
sometimes the pale milky blue tint and aggregate polarisation so often found 
in chlorite. Zeolitic fibrous tufts occur in some of the cavities. I have not 
observed any apatite. 
B. Felspar-Magnetite Rocks. 
Tue Porpuynrites. (Plate XII. figs. 7 and 8.) 
Under this title I provisionally group those Carboniferous volcanic rocks 
which in the Basin of the Firth of Forth have been mapped as “ felstones,” 
“porphyrites,” and “claystones.” They present very great varieties of external 
aspect, but possess certain common characters which suffice to enable the 
field-geologist to distinguish them from the basalt series. Occupying a definite 
place in the Carboniferous system in Scotland, they belong entirely to the 
great volcanic epoch at the beginning of the Carboniferous period. They form 
the thick-terraced masses which range through the north of Ayrshire, Renfrew- 
shire, and Dumbartonshire to the Forth at Stirling. They partially appear at 
Edinburgh, in the Calton Hill and Arthur Seat, but on a much more extended 
scale in the Garlton Hills of Haddingtonshire. Similar rocks in Berwickshire, 
Roxburgh, and Dumfries spread over wide areas at the base of the Calciferous 
Sandstones. They are thus the oldest and most generally distributed of the 
volcanic rocks associated with the Carboniferous system in Scotland. In most 
essential characters they agree with the lavas so copiously erupted in central 
Scotland during the time of the Lower Old Red Sandstone, with which in 
another memoir I shall again discuss them. They thus belong essentially to 
an older as well as a more vigorous volcanic type than the basalts. Even 
the thickest and most extensive series of basalt eruptions, such as those of 
the Burntisland and Linlithgowshire districts, are of trifling amount when 
compared with the enormous sheets of bedded porphyrite in the Campsie and 
Ayrshire Hills. 
The porphyrites always occur as contemporaneous or interbedded sheets, save 
in those comparatively infrequent cases where they have filled up the volcanic 
funnels, and now appear in necks. 
1. General External Characters.—A “porphyrite” is marked by a dull 
close-grained porphyry base, through which are usually scattered crystals of a 
triclinic, less commonly an orthoclase, felspar. The base is usually of some shade 
of red or brown, varying from a dark chocolate or purple tint to pale yellow or 
nearly white, greenish and bluish shades being less common. It is frequently — 
amygdaloidal. As arule, the porphyrites are somewhat altered, fresh specimens 
being in many cases unobtainable, or only with much difficulty. The weather- — 
