﻿Vol. 64.] THE ST. DAYId's-HEAD ' HOCK-SERIES.' 295 



two varieties of augite, plagioclase-felspar, quartz, and micropeg- 

 matite ; they had arisen at three different epochs from three distinct 

 British magmas. They occurred in the Ordovician of Wales, in the 

 late Carbouiferous of Scotland and the North of England (Whin Sill), 

 and as Tertiary intrusions in Arran, Argyllshire, and other parts of 

 the West of Scotland. Many of the features of the Welsh rocks 

 which the Author had described were repeated in the most curious 

 fashion in the Scottish quartz-diabase sills. The older augite of these 

 rocks was a very interesting mineral ; it had the pleochroism and 

 many of the characters of hypersthene, but was monoclinic with an 

 extinction-angle of about 40°. Dr. Wahl had recently published an 

 investigation of this mineral (sahlite, magnesium-diopside, enstatite- 

 augite), showing that it had often a very small axial angle and 

 exhibited great variations in this respect. In the Scottish rocks 

 the speaker's experience was that the axial angle of the older augite 

 was smaller than that of the second augite, but often not much less. 

 Becently, however, on examining sections of the Whin Sill from 

 Belford (Northumberland), he had observed that the sahlite, which 

 was very abundant in that rock, was often uniaxial, an example of 

 a mineral of monoclinic symmetry, optically uniaxial, and with its 

 single optic axis making an angle of 40° with its principal crystallo- 

 graphic axis. 



Mr. J. xiLLAN Thomson" mentioned, as a confirmation of the hypo- 

 thesis that the previous speaker had put forward, the demonstration 

 by Prof. Yogt that enstatite and diopside formed a discontinuous 

 series of mixed crystals. This could be more easily understood if 

 the enstatite entering into the diopside-molecule were monoclinic. 

 Prof. Vogt had placed the enstatite-diopside series in Type IV of 

 Boozeboom's types of mixed crystals, one of his arguments being 

 that enstatite was never posterior to diopside in crystallization. 

 They had seen, however, in the Author's figures a plate of enstatite 

 poecilitically enclosing augite. 



Dr. F. H. Hatch enquired as to tlie nature of the felspars, 

 whether they were anorthoclase or albite. 



Mr. E. B. Bailey remarked that the Scottish Permo-Carboniferous 

 quartz-dolerites showed an exactly similar association of basic, 

 intermediate, and ' acid' types, the dark and light patches frequently 

 occurring side by side, without any evidence of flow-structure, 

 indicating differentiation in situ. The principle involved appeared 

 to be the immiscibility of the extreme products at a temperature 

 slightly above the consolidation-point of the rock. The speaker 

 drew attention to the analyses of the three types, by Mr. G. S. Blake, 

 quoted by Dr. Falconer in his excellent paper on the petrology 

 of the Bathgate Hills, which furnished a very perfect example 

 of ' straight-line ' variation, thus strengthening the Author's 

 conclusions. 



Mr. H. H. Thomas observed, with reference to the age of the 

 St. David's-Head rocks, that they were clearly intrusive into 

 Arenig sediments. In West Pembrokeshire there were several 

 horizons in the Ordovician System marked by volcanic activity, but 



