1915-16.] Trachytes of Clyde Carboniferous Lava-Plateaus. 291 
microcline striation. It appears in the albite-bostonite of Uamh Capuill, 
South Bute, along with small microphenocrysts of albite which have broad 
marginal zones of orthoclase, and an occasional large phenocryst of albite. 
It also occurs in a few trachytic dykes from the March Burn, Kilsyth Hills, 
and from the Misty Law district of Renfrewshire. 
The groundmass of these rocks generally consists of well-shaped felspar 
laths, most of which are zonal, or have multiple twinning, which in suitable 
sections give maximum extinctions ranging in different rocks from 16° to 
7°. As the refractive index is well below that of Canada balsam, these 
figures indicate albite ranging in composition from AbjAn 0 to Ab 95 An 6 . 
There is always a subordinate quantity of non-zonal untwinned laths with 
straight extinction and refractive index less than that of albite, which are 
referred to orthoclase. When the orthoclase increases in amount so as to 
equal or exceed the albite, these rocks become true bostonite, trachyte, or 
keratophyre (see later). Quartz is a constant and sometimes abundant 
constituent of the groundmass. It frequently forms small plates which 
ophitically enclose the felspar laths. 
Ferromagnesian constituents only occur in very small amounts, and 
rarely in a fresh condition. Magnetite in small specks is a constant con- 
stituent, but whatever ferromagnesian minerals have been present have 
almost invariably gone over to chlorite and limonite, rarely even leaving 
definite pseudomorphs. In some cases there may have been a little green 
pyroxene (Neilston Pad). Pseudomorphs in haematite after biotite are 
common in the Great Cumbrae rocks, but fresh biotite was met with in 
only one specimen, from the bostonite north of Keppel Pier. 
In some of the keratophyric and trachytic types there is a little crypto- 
crystalline matter in the groundmass, and this is frequently in process of 
replacement by haematite. Sharply bounded haematised patches formed in 
this way occur in an albite-bostonite lava from east of St Blane’s Hill, 
South Bute, and give the rock an appearance of having picked up angular 
inclusions. Occasionally the groundmass has been partly replaced by 
chalcedonic silica. 
The texture of these rocks varies from the comparatively coarse 
bostonitic varieties to the dense, almost cryptocrystalline keratophyres. 
As regards the mode of arrangement of the felspathic constituents, the 
bostonites generally show a rude fluxional arrangement which may 
occasionally become very perfect, as in the dyke from north of Keppel 
Pier, Great Cumbrae. On the other hand, the laths may be quite diverse 
and unorientated, thus giving the rocks a felted or pilotaxitic texture.. 
The rocks designated as trachytes are of much finer grain than the 
