510 PROFESSOR GEIKIE ON THE 
imperfect, and in irregular shred-like particles. Its crystals are often extremely 
minute ; in some of the rocks of the Garlton Hills they are on an average less. 
than ‘001 of an inch in diameter. In these microscopic grains, however, we 
can easily see with reflected light the glancing triangular faces of the octohedra, 
and the sub-conchoidal fracture of the broken grains. 
Augite is frequently, but not always, present. When recognisable, its most 
common form is that of minute globules, like those of the basalts, but of still 
smaller dimensions, enclosed in the felspars and in the clear ground-mass. 
Larger irregularly defined fragments, of a pale yellow tint in thick sections, 
which polarise like the augite globules, are probably also augite. 
At the base of the lavas of the Garlton Hills certain rocks occur which 
present the same peculiar anisotropic ground-mass, but in much smaller 
quantity. The triclinic felspars are numerous, fresh, and well striated. Augite 
abounds in large crystals, as well as in smaller globular forms. Magnetite or 
titaniferous iron appears, but has commonly suffered oxidation. Pseudomorphs 
of serpentine and black ferruginous opaque matter replacing olivine (7?) like- 
wise occur. These rocks evidently form a connecting group between the 
anamesites and porphyrites. 
The most characteristic Carboniferous porphyrites in the Baia of the 
Firth of Forth are those of East Lothian. They have been laid open in — 
numerous quarries, as well as natural sections, on the Garlton Hills. The 
Calton Hill and Arthur Seat porphyrites are much decayed, but still show 
the characteristic ground-mass. The Campsie Fells contain many varieties of 
porphyrite, but these lie chiefly beyond the region embraced by the present — 
Memoir. 
In the progress of alteration the porphyrites undergo some characteristic 
changes. Their felspar suffers the usual kaolinisation. Their ferruginous 
constituent is oxidised into hematite, but more usually limonite, and they con- _ 
sequently weather into reddish-brown and yellow clays. 
C. Felspar (Orthoclase) Rocks. 
THE FeusiTes. (Plate XII. fig. 9.) 
Very few rocks of this class are included among the Carboniferous volcanic 
masses of the Basin of the Firth of Forth. Some examples occur among the 
necks of the Campsie Fells, and in the western neck on the shore at Largo. 7 
1. General External Characters and Modes of Occurrence.—Under the term 
felsites, I group certain rocks varying in colour from a pale grey through shades 
of yellow to a deep red, usually compact in texture; in fresh fracture some- 
times quite flinty, but mostly decayed, and presenting a more or less kaolinised © 
