It is probable these last four specimens come from a stratum more nearly allied to the gypsum formation of Paris than to the red marl to which I have referred the other specimen of gypsum, No. 12.
magnesian limestone, or dolomite.
17. Limestone, very compact, and strongly impregnated with magnesia, and is of a yellowish white colour; occurs split into small laminated fragments, like broken tiles. The ground is covered with these fragments, which break and rattle under the feet like pottery. It is found in the plain between Zuela and Omesoguin, lat. 26°.
18 and 19. From the plain that lies between the Black Mountains and Pass of Kenair. Both these specimens are compact limestone, of a dark flesh colour, and contain much magnesia. This plain is covered with small shining fragments of this limestone, particularly near El Gaaf; and their exposed surfaces are corroded and furrowed over with the small irregular grooves and channels already mentioned in the note to No. 1.
20. Flesh-coloured dolomite, mottled with yellow. From the north base of the Black Mountains.
21. Hard compact dolomite, of an ochre yellow colour. From the Well of Gutfa, between the north base of the Black Mountains and town of Sockna.
22 and 23. Brecciated dolomite, of yellowish white colour, full of knobs and concretions of the same. Found on the plain near Sockna.
24 and 25. Brecciated dolomite, of dark flesh colour, full of small angular concretions of the same substance, but in more compact state than that which forms the cement by which they are held together. These are found near the compact flesh-coloured dolomite, No. 18 and 19, in the plain of El Gaaf.
All the above specimens, from 17 to 25, are much impregnated with magnesia, and distinctly referrible to similar strata that form subordinate beds in the new red sand-stone formation of England; and they seem to occupy a similar position in the red sand-stone of the desert plains that lie on the north and south side of the basaltic chain of the Black Mountains in Fezzan.
26. A pudding-stone, composed of pebbles of dolomite, united by a granulated calcareous cement. Taken from a Roman castle at Bonjem. The surface of this specimen has the same glossy polish which has been before mentioned in some others.