378 W. H. HUDLESTON ON THE CHEMICAL 



very black. There is evidently an amount of carbon, which tends 

 to darken the colour of the rock. 



No. III. Another specimen of unaltered Trilobite-flag. It is 

 much darker in colour than No. I., and does not effervesce in the 

 cold. It is thin-bedded, and has Trilobite-remains on both sides, por- 

 tions of the shell being in some eases preserved. There is much 

 pyritization in the vicinity of the fossil ; and the partial oxidation 

 produces seams and stains of ferric hydrate, especially about the 

 furrows between the costae. The portion underneath the glabella 

 is a honey- combed mass of partially oxidized pyrites. Specific 

 gravity 2*77. There is a large amount of phosphoric acid in the 

 outer skin of this specimen, but hardly any lime carbonate. This 

 shows that phosphates and lime carbonates are not necessarily inse- 

 parable. There is, however, a considerable amount of ferrous oxide, 

 which may perhaps have replaced the lime. The qualitative analysis 

 shows sulphur in great quantities, and traces of manganese. 



No. Ill *. " Shell with a little matrix." This is a still more 

 superficial scrape from a specimen similar to the last. It contains 

 large quantities of phosphoric acid, as the subjoined partial analysis 

 will show. 



per cent. 



Insoluble, much pyrites 30*85 



Phosphoric anhydride 17*05 



Lime ; . abundant. 



Ferrous oxide . . abundant. 



Magnesia not much. 



The phosphates in this specimen, calculated as ordinary calcic 

 phosphate, amount to more than half the soluble matter ; it is not 

 improbable that some chloritic, glauconitic, or serpentinous silicate 

 is also associated. There are indications of this likewise in Nos. I. 

 and II. 



No. IV. " Trap." Fragment from the core. Distinctly crystal- 

 line. Felspar crystals ; small twins of orthoclase. The amphibolic 

 element is in a decomposed and hydrated condition. It contains 

 patches and stains of ferric hydrate, and little irregular vacant spaces 

 (not vesicles, however), is easily crushed and evidently far advanced in 

 decomposition. Crystals of pyrites visible. No effervescence in the 

 cold. Specific gravity 2*59. This is less than that of any of the sedi- 

 mentary rocks, I., II., or III., and is all the more remarkable as the 

 specimen contains nearly 10 per cent, of the oxides of iron. The un- 

 dried powder yields to hydrochloric acid 25 per cent. ; and if we add an 

 equal amount of silica in combination, half the rock is attacked by 

 acid. Of the soluble bases there is 7*71 per cent, ferrous oxide to 1*55 

 per cent, ferric oxide. The rock loses much water on ignition ; it 

 contains (in the soluble bases) but little lime, though large quantities 

 of magnesia. All these facts point to a serpentinous condition of 

 a large portion of this trap. The partial fusion of the insoluble 

 residue shows both soda and potash. 



per cent. 

 Phosphoric anhydride 0-323 



