1909-10.] 
The Glenboig Fireclay. 
359 
6. The Formation of the Glenboig Fireclay. 
The characters of the Glenboig sideroplesite show that this mineral 
grew in situ at the time of the deposition of the clay and not by subsequent 
segregation like the whinny boles of iron carbonate in our Carboniferous 
sandstones. The centres of the crystals are often quite transparent and 
water clear, and they were doubtless formed in the water of the lagoon ; 
the outer layers were probably added after the crystals had fallen on to 
the floor, as they contain some mechanically-included mud. 
This view is supported by the facts that no such crystals have ever 
been described from the English underclays, and that a somewhat similar 
occurrence of dolomite in the Keuper Marl, discovered by Dr C. G. Cullis,* 
has been attributed by him to precipitation from the waters of an inland 
sea or lake. He has described numerous minute crystals of dolomite 
in the Keuper Marl from Westbury-on-Severn. These dolomite crystals are 
perfect in shape and have the form of the fundamental rhombohedron, but 
they are far more minute than the sideroplesite of Glenboig. Dr Cullis 
attributes the formation of these dolomite crystals to precipitation from 
solution in the waters of an inland sea or lake while the marl was being 
precipitated from suspension. 
The existence of the sideroplesite, the angularity of the included sand 
grains, and the vertical position of some of the flakes, all indicate that the 
Glenboig fireclay was formed in still water. The absence of marine 
organisms precludes the idea that the formation was marine, for had fossils 
ever been present, they should have been excellently preserved in this 
compact clay. There are no fresh-water shells in the deposit. It seems 
to me, therefore, most probable that this fireclay originated in a great 
lagoon. The “ Roman cement ” below was a marine formation, for it is 
often crowded with marine shells. Mr Hinxman records the fact that 
the cement stone to the east of Glenboig contains abundant Orthotetes 
crenistria.\ Above this marine limestone occurs a clay-band ironstone, and 
then follow shales and sandstones, followed by the fireclay. The fireclay, 
therefore, probably marks a period during the deposition of these beds in 
which the progressive emergence of the land led to part of the sea being 
cut off as a wide lagoon. Into this lagoon water from the adjacent land 
carried soluble bicarbonates of iron, magnesia, and lime ; the loss of part 
of the carbonic acid reduced the bicarbonates to the insoluble unicarbonates, 
* C. G. Cullis, “ On a Peculiarity in the Mineralogical Constitution of the Keuper Marl, :> 
Rep. Brit. Assoc. Leicester , 1907, p. 507. 
t Summary of Progress, 1907, p. 103. 
VOL. XXX. 
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