348 
Proceedings of the Royal Society of Edinburgh. [Sess. 
XVIII. — The Glenboig Fireclay.* By J. W. Gregory, D.Sc., F.R.S. 
(L. and Ed.), Professor of Geology in the University of Glasgow. 
(With One Plate.) 
(MS. received February 24, 1909. Read March 15, 1909.) 
CONTENTS. 
1. The Nature of Fireclay 
2. The Glenboig Fireclay 
3. The Clay Substance ...... 
4. Materials included in the Clay 
5. The Sideroplesite 
6. The Formation of the Glenboig Fireclay . 
7. Summary of Conclusions 
PAGE 
348 
350 
352 
355 
356 
359 
360 
1. The Nature of Fireclay. 
According to Percy’s definition, *f- “ clays are termed fireclays or refractory 
clays when they resist exposure to a high temperature without melting or 
becoming in a sensible degree soft and pasty.” The refractoriness of fireclay 
is due to its low proportions of fluxes. The best known British fireclays 
occur beneath the coal seams of the coal measures, and are therefore known 
as “ under-clays ” or “ seat-clays.” The explanation of their paucity of 
fluxes usually offered is that the alkalies have been withdrawn as food by 
the vegetation which formed the coal. Thus Professor Tarr J remarks that 
fireclays “ are particularly abundant in the Carboniferous rocks associated 
with coal beds, the plants having been instrumental in the withdrawal 
of the alkalies.” This view has been widely accepted, and the death of 
the successive coal forests has been attributed to the exhaustion of the plant 
foods in the underlying soils. Caution is, however, necessary in the applica- 
tion of this theory ; for plants can only withdraw soluble alkalies from the 
* I must express my indebtedness for help during several visits to Glenboig to the late 
A. H. Dunnacliie, the 'General Manager of Glenboig Union Fireclay Co. Ltd. ; Mr J. 
Macintyre, the mine manager ; also to Mr G. W. Tyrrell of the Glasgow University for the 
preparation of slides and determination of the specific gravity, and to Mr D. P. Macdonald, 
the Baxter Demonstrator in Geology, for the analysis of the sideroplesite. Also to Professor 
A. C. Seward of Cambridge and Dr J. W. Evans of the Imperial Institute for examining 
one of the slides, and to Mr Fingland for the photographs. 
t J. Percy, Metallurgy , Fuels , 1875, p. 87. 
f R. S. Tarr, Economic Geology of the United States, with Briefer Mention of Foreign 
Mineral Products, 1894, p. 400. 
