420 SECTIONAL TRANSACTIONS— C. 



by the early crystallisation and removal of olivine. The siliceous and 

 potassic residuum crystallised at temperatures above the leucite-orthoclase 

 reaction temperature and v/as extruded as a crystal mush which has effected 

 little change in the intruded sediments. Chilling of the acid residuum has 

 inhibited the leucite to orthoclase reaction. 



The only comparable rocks are the wyomingites and orendites of the 

 Leucite Hills, Wyoming. 



Dr. F. Coles Phillips. — The fabric of some 'Tarskavaig Moines' (12.20), 



The ' Tarskavaig Moines,' a series of phyllites and schistose grits, occur 

 in the Sleat district, Skye, to the north-vilest of the Moine Thrust, above a 

 subsidiary dislocation, the Tarskavaig Thrust. On the Geological Map of 

 Scotland issued in 1892 they were coloured as Torridonian. They were 

 later regarded by C. T. Clough (mainly because of supposed similarities ia 

 stratigraphical sequence) as less-altered representatives of the same great 

 formation as the Moine rocks on the other side of the Moine Thrust. This 

 view has recently been questioned by H. H. Read, who considers the 

 Tarskavaig rocks to be Torridonian affected by the post-Cambrian disloca- 

 tions, and regards apparent transitions from unaltered Torridonian to 

 true Moine rocks as a result of metamorphic convergence. The fabric 

 has therefore been examined in an attempt to determine the direction or 

 directions of movements to which these rocks have been subjected, and 

 comparisons are instituted between the grain-fabric of the Tarskavaig 

 rocks and of Torridonian and Moine rocks from adjacent districts. These 

 are believed to show that true Moine rocks have been affected by regional 

 movements along a south-west and north-east direction, no trace of which 

 can be found in the grain-fabric of the Tarskavaig rocks. 



Afternoon. 

 Excursion to Barley and Barkway. 



Dr. S. R. NocKOLDS and Dr. J. E. Richey, F.R.S. — Replacement veins in 

 the Mourne Mountains granites (2.15). 



All the Tertiary granite masses of the Mourne Mountains are traversed 

 here and there by narrow replacement veins which belong, in general, to 

 the greisen class ; the prevalent variety being dark-green in colour. They 

 are either steeply inclined or vertical, are found in greatest number at places 

 close to a granite margin, and can be shown in several instances to be 

 parallel to the plane of contact. Their position in the igneous history of 

 the granites is indicated by the fact that they cut the later aplite veins but 

 are themselves traversed by well-developed joint planes. 



The dominant type of vein is composed of aggregates of topaz and a 

 peculiar blue-green mica together with quartz. Other constituents include 

 some independent topaz, fiuorite, stilbite, chlorite and sometimes a little 

 biotite. 



Subordinate types are represented by white mica-chlorite-quartz veins 

 with minor fluorite and stilbite, grey quartz-magnetite veins and black veins 

 rich in manganese ores. 



There occur, in addition, certain fissure veins which follow, and are later 

 than, the joints. The dominant variety is composed of iron rich chlorite, 

 associated with colourless granular fluorite, albite and very subordinate 

 orthite. 



