356 Proceedings of the Royal Society of Edinburgh. [Sess. 
are occasional crystals of zircon and abundant excessively small needles and 
granules of rutile. 
Hydrated iron oxide occurs in grains and in occasional segregations. 
5. SlDEROPLESITE. 
The most interesting mineral included in the Glenboig fireclay is a 
hexagonal carbonate with a well-defined zonal structure and at first sight, 
in sections, a strikingly organic appearance. The zonal structure (figs. 1 
and 2) combined with the radial arrangement of the particles gives the 
crystals a general resemblance to some calcareous algse. I accordingly sent 
the first slide in which these crystals were found to Professor A. C. Seward, 
F.R.S. He could not accept them as calcareous algge, and kindly referred 
me to Karpinsky’s monograph on the Trochiliscidae as containing photo- 
graphs of the calcareous algae most similar to these structures.* 
The sections show that these bodies are zonal crystals of a hexagonal 
carbonate belonging to the series of dolomite and siderite. They are in the 
form of rhombohedra with curved faces. Dr J. W. Evans kindly examined 
one of the slides, and remarked that the regular coincidence of the axes of the 
lozenge-shaped sections with the optical axes was inconsistent with their 
organic origin. The material of a fossil may, of course, be crystallised ; 
but the direction of the extinctions in these crystals shows that their 
external form was an original structure. I can see no evidence that their 
zonal structure is a later development. 
These crystals occur in abundance on certain layers ; as, for example, in 
the clay known as the “ compound,” above the valuable fireclay beside the 
Gartverrie Haulage in the Star Mine. A seam full of these crystals was 
passed through at a depth of 59 feet in bore No. 5, described in the bore 
journal as 5 feet of “ coarse dark fakes.” f These crystals are recognisable 
as coarse grains by the naked eye, and that these are not quartz grains can 
be recognised by their softness. 
Some of them were separated by Mr G. W. Tyrrell, who determined 
their specific gravity as 3 - 63, which is appreciably lower than that of 
siderite, which is given by Miers as 3*86 and by Dana as 3 7 to 3"9. For 
the following analysis I am indebted to Mr D. P. Macdonald, the Baxter 
Demonstrator in Geology in the University. 
* A. Karpinsky, “ Die Trochilisken,” Mem. Comite' Ge'ol., new ser., No. xxvii., St 
Petersburg, 1906, viii + 172 pp. ; 3 pi., 59 figs, (in Russian, with German summary); see, 
e.g., fig. 7, p. 12 ; fig. 10, p. 18 ; fig. 74, p. 82 ; and pi. 3, fig. 2. 
t Fakes is a local term for laminated sandy clay or sandy shale. 
