194 
PROFESSOR T. J. JEHU AND DR ROBERT CAMPBELL ON 
whole, we are justified in fixing the horizon of the Lower or Black Shale and Chert 
Series as Upper Cambrian or the Passage Beds between the Cambrian and the 
Ordovician. 
In this connection it may be noted that Dr Peach, in his Presidential Address 
to the Geological Section of the British Association at the Dundee meeting in 1912, 
expressed the opinion that Upper Cambrian strata will yet be found in the Girvan 
area — a view, he adds, which had been previously suggested by Professor Lapworth.* 
It follows that the Upper or Margie Series, which is unconformable to the 
Lower Series, must belong at any rate to later Ordovician times. Here again the 
resemblance to the succession in some parts of the Southern Uplands still holds, for, 
in the Girvan area, on the north side of the Stinchar Valley, grits, shales, and 
conglomerates overlie unconformably the cherts, black shales, and volcanic rocks of 
the Lower Series, and the conglomerates include fragments of the underlying rocks, t 
The presence of the remains of Crinoids and of other organisms in the Aberfoyle 
Limestone of the Upper Series affords additional and conclusive evidence of the later 
Ordovician age of the series. The occurrence of calcareous Algae and Foraminifera 
with the remains of Crinoids suggests a correlation of this Limestone with 
the Stinchar and Craighead Limestones, which likewise contain calcareous Algae , 
Foraminifera, and occasional crinoidal remains.^ A further search may result in the 
discovery of other organisms in the Aberfoyle Limestone. 
VIII. Intrusive Bocks older than the Lower Old Red Sandstone. 
These rocks include albite diabase, albite hornblende gabbro, and serpentines. 
An albite diabase intrusion occurs on the watershed to the north-west of Gualann, 
and others outcrop at intervals along the ridge of black shales and cherts as far 
east as a point south-east of Garbeg Hill. An intrusive rock, shown on the accom- 
panying map as albite diabase, but which is so highly decomposed that its affinities 
are somewhat uncertain, is seen on the left bank of the Bofrishlie Burn east of 
Dungarrow. Diabases, in all probability intrusive in character, are associated with 
the spilites where they first make their appearance west of the Ivelty Water, but 
they have not' been mapped separately, and they are so highly altered that it has 
not been found possible to determine their systematic position. 
The serpentines and gabbros form part of a complex of igneous, sedimentary, 
and vein rocks which can be traced almost continuously from the old limestone 
quarry north of Upper Dounans to Gualann. The most interesting exposures occur 
in and to the south-west of the above-mentioned quarry and in the neighbourhood 
of Maol Ruadh and Lime Hill. 
Petrography of the Albite Diabase. — The freshest specimens were obtained from 
the large intrusion opposite Gualann. They consist chiefly of the three minerals 
* Peach, Rep. Brit. Assoc., p. 453, 1912. 
f The Silurian Rocks of Britain, vol. i, Scotland (Mem. Geol. Survey), pp. 45-47, 1899. I Ibid., p. 46. 
