Minerali^leal Defcrlpt'wn of Cihrahaf, jij 



The ftalaftites of thcfe caves when near the furface of the moiintain are of a brownlfli yellow 

 colour ; but as we defccnded towards the lower caves we found them begin to lofe their 

 darknefs of colour, which by degrees fhaded off to a whiiifti yellow. 



The only inhabitants of thefe caves are bats, fome of which are of a large fize. The foil 

 in general upon the mountain of Gibraltar is but thinly fown ; and in many parts that thin 

 covering has been waflied off by the heavy autumnal rains, which have left tha fuperficies 

 of the rock for a confiderable extent bare and open to infpedlion. In thoft fituations an 

 obferving eye may trace the effefts of the flow but cotiftant decompofition of the rock, caufed 

 by its expofure to the air, and the corrofion of fea falts, which in the heavy gales of eafter- 

 ly winds are depofited with the fpray on every part of the mountain. Thofe uncovered parts 

 of the mountain rock alfo expofe to the eye a phenomenon v7orthy of fome attention, as it 

 tends clearly to demonftrate, that, however high the furface of this rock may now be elevated 

 above the level of the fea, it has once been the bed of agitated waters. This phenomenon 

 is to be obferved in many parts of the rock, and is conftantly found in the beds of torrents. 

 It confifts of pot-like holes of various fizes hollowed out of the folid rock, and formed ap- 

 parently by the attrition of gravel or pebbles fet in motion by the rapidity of rivers or currents 

 in the fea'. One of thofe, which had been recently laid open, I examined with attention. I 

 found it to be five feet deep and three feet in diameter ; the edge of its mouth rounded ofFas 

 if by art, and its fides and bottom retaining a confiderable degree of polifli. From its mouth 

 ibr three feet and a half down it was filled with a red argillaceous earth, thinly mixed with 

 minute parts of tranfparent quartz cryflials : the remaining foot and a half'to the bottom 

 contained an aggregate of water-worn (tones, which were from the fize of a goofe's egg to^ 

 that of a fmall walnut, and confided of red jafpers, yellowifh white flints, white quartz, and 

 blueilh white agates, firmly combined by a ycllowifh brown ftaladlitical calcareous fpar. la 

 this breccia I could not difcover any fragment of the mountain rock, or any other calcareous 

 matter, except the cement with which it was combined. This pot is 940 feet above the 

 level of the fea. 



Upon the weft fide of the mountain towards its bafe fome ftrata occur, which are hete- 

 rogenial to the mountain rock : the firfl; or highefl forms the fcgment of a circle j its con- 

 vex fide is towards the mountain, and its flopes alfo in that direiSlion. This ftratum confifts 

 of a number of thin beds ; the outward one, being the thinneft, is in a fl:ate of decompofition, 

 and is mouldering down into a blackifh brown or ferruginous coloured earth. The beds 

 inferior to this progreflively increafe in breadth to 17 inches, where the flratification refts 

 upon a rock of an argillaceous nature. 



This laft bed, which is 17 inches thick, confifls of quartz cf a blackifh blue colour, in 

 the fepta or cracks of which are found fine quartz cryftals, colourlefs and perfedlly tranf- 

 parent. Thefe cryftals are compofed of 18 planes, difpofed in hexangular columns, termi- 

 nated at both extremities by hexangular pyramids. The largeft of thofe that I have feen 

 does not exceed two-eighths of an inch in length : they in general adhere to the rock by the 

 fides of the column, but are detached without difficulty. Their great degree of tranfparency 

 has obtained them the name of Gibraltar diamonds. 



[To be continued.'] 



Bb2 JG. 



