360 Proceedings of the .Royal Society of Edinburgh. [Sess. 
and thus led to their precipitation and growth into these crystals of 
sideroplesite. 
Continued emergence of the land led to the site of the lagoon being 
covered by a sheet of sand, which is now consolidated into the sandstones 
overlying the fireclay. This sand was in time covered by soil and by trees, 
whose roots exist as the Stigmaria in the sandstone, and whose remains 
form the thin coal seam above the sandstone. During the filling of the 
lagoon with sand strong streams flowed across the locality, cutting channels 
in the clay. These stream valleys were subsequently filled by sand and 
thus gave rise to the “ bellies ” of sandstone that occasionally cut out the 
fireclay and bring the sandstone of the roof down to the floor of the 
clay seam. 
7. Summary of Conclusions. 
1. The Glenboig fireclay was a lagoon deposit in the lower part of the 
Millstone Grit Series. 
2. Its clay substance is an amorphous hydrous bisilicate of alumina, 
and may be regarded as the mineral halloysite ; much clay substance is 
referable to this material. Other clay substances are kaolinite, sericite, 
and quartz-flour. 
3. The fireclays contain zonal, lenticular, and rhombohedral crystals of 
sideroplesite which grew in the water and on the floor of the lagoon. 
(Issued separately April 8 , 1910 .) 
