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. 27. Specimen of the materials of which the Castle of Zuela and many other old castles are constructed. It is an artificial compound, made up of small grains of quartz of the size of millet seed, imbedded in a cement or paste of yellowish marl, which effervesces rapidly with acids: there is no more marl than is sufficient to hold together the grains of quartz. Buildings are constructed of this material by pressing the composition into cases, which are removed when it is dry. 28. Milk-white compact carbonate of lime. From the plain between the Black Mountains and Pass of Kenair. It is beautifully furrowed over with small channels and grooves, like No. 18 and 19. 29. White limestone, of coarser grain. From ditto. 30. White limestone, filled with grains of fine white quartz. From ditto. 31. White limestone. From the north base of the Black Mountains. 32. Compact yellow carbonate of lime, having a polished glossy surface, beautifully furrowed, and resembling a bit of yellow bees’ wax. From the north base of the Black Mountains. These last specimens of carbonate of lime are found with those of dolomite described immediately before, and seem to belong to the same formation with them.