Reports & Proceedings—Geological Society of London. 237 
differ in certain respects from a normal magma of granitic composition, 
and it is thought most likely that the veins represent the solid 
equivalents of such a melt. 
4. April 5, 1916.—Dr. Alfred Harker, F.R.S., President, in the Chair. 
The President announced that the Council had awarded the proceeds 
of the Daniel Pidgeon Fund for the present year to John Kaye 
Charlesworth, M.Sc., Ph.D., F.G.8., who proposes to conduct 
researches in connexion with the Glaciation of Donegal. 
The following communication was read :— 
“The Picrite-Teschenite Sill of Lugar (Ayrshire) and its Differ- 
entiation.””. By George Walter Tyrrell, A.R.C.Sc., F.G.S. 
This sill occurs near the village of Lugar in East Central Ayrshire, 
and is magnificently exposed in the gorges of the Bellow and 
Glenmuir Waters, just above the confluence of these streams to form 
the Lugar Water. It has a thickness estimated at 140 feet, and 
is intrusive into sandstones of the ‘ Millstone Grit’. The contacts 
consist of a curiousiy streaked and contorted basaltic rock, passing 
at both margins into teschenite. The upper teschenite, however, 
becomes richer in analcite downwards, and ends abruptly at a sharp 
junction with fine-grained theralite. The lower teschenite becomes 
somewhat richer in olivine upwards, but passes rapidly into horn- 
blende-peridotite. The central unit of the sill is a graded mass 
beginning with theralite at the top and passing gradually into 
picrite, and finally peridotite, by gradual enrichment in olivine and 
elimination of felspar, nepheline, and analcite. 
The field detail of the Bellow, Glenmuir, and other sections is 
given in part ii of the paper; and the petrographic detail, with 
several chemical analyses, in part ii. A unique rock, named 
lugarite in 1912, with 50 per cent of analcite and nepheline, 
occurs as an intrusion into the heart of the ultrabasic mass of the 
sill. Part iv deais with the special significance of this sill in 
petrogenetic theory. The mineral and chemical variations are 
described and illustrated by diagrams. It is shown that the average 
rock of the sill, obtained by weighing the analyses of the various 
components according to their bulk, is much more basic than the 
rock now forming the contacts. Hence, assuming that the sill is 
a unit and represents a single act of intrusion, the main differentiation 
cannot have occurred in situ. Other special features of the sill are 
the identity and banding of the contact rocks, its asymmetry, the 
density stratification of the central ultrabasic mass, and the sharp 
junction between the upper teschenite and the underlying theralite. 
The theory is advanced: that the differentiation units were produced 
by the process of liquation, but that their arrangement within the 
sill took place under the influence of gravity. There are sharp 
interior junctions between a unit consisting mainly of calcic ferro- 
magnesian silicates and a unit consisting mainly of alkali-alumina 
silicates with water, the former giving rise to the central ultrabasic 
stratum and the latter to the teschenites. These partly immiscible 
