188 Proceedings of Royal Society of Edinburgh, [april 4, 
the adjacent Silurian rocks. If such derivative matter occur at all 
it cannot he common, or I should hardly have missed it. 
Intrusive Rocks . — Intrusive rocks, as already remarked, are well 
developed in the headland of St Abb’s. These consist principally 
of a compact fine-grained blue porphyrite, which is often reddened 
with ferritic matter. It is a hard, tough rock, very irregularly 
jointed. Here and there it is sparingly porphyritic with “ferrite” 
— evidently pseudomorphous after pyroxene. The rock is a good 
deal weathered, and fresh specimens are not readily obtained. Under 
the microscope it shows a crypto-crystalline ground-mass — apparently 
composed entirely of small crystals of plagioclase, hut owing to 
weathering the characteristic form and striation of the crystals are 
not well seen. There is no trace of non-individualised basis — 
nothing resembling the vitreous and devitrified basis of the bedded 
porphyrites. Scattered through this ground-mass occur occasional 
large crystals of plagioclase, and here and there more or less abundant 
pseudomorphs of limonite and serpentine after hornblende or 
pyroxene — the latter apparently being most common. Diallage, 
partially altered into limonite and serpentine, appears now and 
again ; and probably many of the pseudomorphs just referred to 
may represent this mineral. Other decomposition-products diffused 
through the base in some sections are secondary quartz and 
chalcedony, and, in thin veins, calcite and haematite or limonite.* 
(b) Rocks of Coldingham Shore and Coldingham Bay. 
The igneous rocks of this area are separated from those of the 
region just described by a narrow belt of vertical Silurian strata 
which occupy the cliffs and foreshore between Halterem’s Loup and 
the Long Carr. At Halterem’s Loup the junction between the Old 
Ked Sandstone and the Silurian is a well-marked fault. The grey- 
wackes are considerably hardened and shattered, more especially as 
they approach the igneous rocks of Coldingham Shore. At one or 
* At Bell Hill the conglomerate is traversed by a dyke of mica-trap or 
minette. This rock shows under the microscope a micro-crystalline ground-mass 
full of felspar microliths and black magnetite dust. Scattered abundantly 
through this ground-mass are small scales and larger crystals of biotite, many 
of which are broken and twisted, and contain inclusions of the ground-mass. 
Orthoclase appears sparingly, and a few irregular crystalline granules of quartz 
are present. Magnetite is very plentiful. 
