Geological Society of London. 45 



their organic remains. The Graptolithina of the Lower or Glenkiln 

 division are those of the highest Llandeilo Flags of Wales, the 

 corresponding Middle Dicranograptus-?,(i\i\^ts of Sweden, and the 

 Norman's Kiln shales that underlie the Trenton (Bala) Limestone of 

 New York. The Hartfell species occur in the Bala beds of Conway, 

 etc., the higher Dicranograptus-SG\i.\s,ts of Sweden, and the Utica 

 and Lorraine shales that overlie the Trenton Limestone. Those of 

 the Birkhill shales agree almost species for species with the fossils 

 of the Coniston Mudstone of Cumberland, the Kiesel Schiefer of 

 Thuringia, and the Lobiferous beds of Sweden, which lie at the 

 summit of the Lower Silurians of their respective countries. Hence 

 it may be considered certain that the Glenkiln shales are of highest 

 Llandeilo age, that the Hartfell shales stand in the place of the Bala 

 or Caradoc of Siluria, and that the Birkhill shales correspond to the 

 Lower Llandovery. 



The insignificant thickness of these three formations in the Moffat 

 district is in strict agreement with the well-known north-westerly 

 attenuation of the Lower Silurian rocks in Wales, England, and in 

 Western Europe generally. 



It was pointed out that these results, when carried to their 

 legitimate conclusion, harmonize all the apparently conflicting facts 

 hitherto collected among the Lower Silurians of the south of Scot- 

 land. We have a complete explanation of such difficulties as the 

 remarkable lithological uniformity of the predominating strata, the 

 absence of associated igneous rocks, the peculiar localization of the 

 fossils, their identity along certain lines, and their rapid and 

 peculiar impoverishment along others. We reduce, at a single 

 stroke, the apparently gigantic thickness of the South Scottish 

 Silurians to reasonable limits, and at the same time bring them into 

 perfect harmony with those of Western Europe and America. 



II.— December 5, 1877.— Prof. P. Martin Duncan, M.B., F.R.S., 

 President, in the Chair. The following communications were 

 read : — 



1. " On the Building-up of the White Sinter Terraces of Eoto- 

 Mahana, New Zealand." By the Rev. Richard Abbay, M.A., F.G.S. 



The author described the structure and mode of formation of the 

 so-called "White Terrace" of the Roto-Mahana, which is produced 

 by a deposit of silica from the water of a geyser situated on the side 

 of a small hill of rotten rhyolitic rock, about 100 feet above the 

 surface of the warm lake (Roto-Mahana), into which the water from 

 the geyser finally flows, and the foot of the siliceous terrace projects. 

 The geyser basin, which is between 300 and 400 feet in circum- 

 ference, has steep walls broken through only on the side towards 

 the lake, where the water flows down to form a succession of 

 terraces, which are really shallow basins, over the outwardly in- 

 clined edges of which the water flows, depositing the dissolved silica 

 in a white subflocculent form on the edges and bottoms of the 

 basins in proportion as the water cools. The author showed how 

 this arrangement produced the peculiarly formed siliceous deposit of 

 the terraces, and that as the growth of the latter is evidently up- 



