■406 Oji the Chemical Composition of Phosphatic Minerals 
limestone from the black phosphatic shale. The vein and ac- 
companying phosphatic deposits run east and west. 
The Ijlack phosphatic slate or band is fully 18 inches thick, 
and the limestone-bed from 8 feet 6 inches to 9 feet. The vein 
which separates the two deposits from each other is 14 to 1(5 
inches wide, and filled partially with white pipe-clay, calcareous 
spar, and copper and iron pyrites. 
Since the discovery of the phosphate mine at Cwmgynen, 
others of a similar character have been found in North Wales; 
and to some extent Silurian phosphate has made its way into 
the hands of manufacturers of superphosphate of lime. The 
proportion of phosphoric acid in the black shale of Cwmgynen 
varies greatly. Towards the summit of the hill it is not 
nearly so rich in phosphoric acid as at a lower level, where 
it appears in compact masses, free from carbonate of lime, 
and containing but little iron pyrites, of which considerable 
proportions occur in the shale from a higher level. 
A specimen taken from a compact block from the lower level 
of the mine, and weighing about 1 cwt., on analysis gave the 
following results :■ — 
Composition of a Sample of Silurian Phosphate. 
Organic matter and loss on heating (chiefly graphite) 3 '98 
*Phosphoric acid ■ .. 29-67 
Lime 37-16 
Magnesia -14 
Oxide of iron 1-07 
Alumina, fluorine, and loss in analysis 5-84 
Insoluble siliceous matter 22-14 
100-00 
* Equal to tribasic phosphate of lime .. .. 64-77 
In this specimen of Silurian phosphatic shale no>iron pyrites 
was visible to the naked eye, and as it contained but little 
iron, iron pyrites cannot have been present in appreciable quan- 
tities. It was also free from carbonate of lime ; and besides 
black carbon, insoluble shale, alumina, and some fluoride of cal- 
cium, it contained the largest percentage of phosphate of lime 
which I ever found in picked samples from the Cwmgynen 
mine. In some places the black shale contains only from 25 to 3C 
per cent, of phosphate of lime. As already stated, it varies_grcatb 
in composition at different depths of the mine. The black shale 
moreover, passes gradually into the phosphatic limestone-bed 
and in places contains from 10 to 15 per cent, of carbonate o 
lime and magnesia. 
