176 Geological Society. 



picrite, and finally peridotite, by gradual enrichment in olivine and 

 elimination of felspar, nepheline, and analcite. 



The field detail of the Bellow, G-lenmuir, and other sections is 

 given in Part 2 of the paper ; and the petrographic detail, with 

 several chemical analyses, in Part 3. 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 4 deals 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 ultra- 

 basic mass, and the sharp junction between the upper teschenite 

 and the underlying theralite. 



The theory is advanced that the differentiation units were pro- 

 duced 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 immis- 

 cible fractions arranged themselves according to density. Then 

 within the central ultrabasic stratum there was a subsidiary 

 gravity-stratification— due to the subsidence of olivine-crystals, 

 giving rise to the graded mass described above. If differentiation 

 had occurred subsequent to the arrival of the sill in the position 

 that it now occupies, the contact-rocks should have the same com- 

 position as the average rock of the sill. This, however, is not the 

 case, as the average rock has the composition of an almost ultrabasic 

 theralite, entirely different from the teschenites of the contacts. 

 Hence it is believed that, after forming contact-sheaths of theralite, 

 and undergoing gravity-stratification subsequent to liquation, the 

 intrusive activity was renewed, and the sill was moved on along 

 bedding-planes into cold rocks, leaving its contact-sheath behind 

 adhering to the old contacts, and establishing new contacts with 

 its upper and lower teschenite-layers. Here crystallization began, 

 and, by the subsidence of olivine, the subsidiary gravity- stratification 

 of the central ultrabasic layer was effected. The extraordinary 

 flow-banding shown by the contact-rocks affords confirmation of 

 the renewed movement thus postulated. 



In conclusion, the sill is compared with five other teschenite- 

 picrite sills in Scotland, those of Ardrossan, Saltcoats, Blackburn, 

 Barnton, and Inchcolm. 



