Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/276

266 vitrified wall, they rejected that which was unfit fer their purposes, however conveniently placed. Had the object been to erect a dry wall of stone and wood the limestone would have equally answered their intentions. This notion of a design to vitrify seems to receive additional strength from the apparent solicitude and labour employed in introducing so much pudding-stone into the work. It is very likely that accident had taught them the vitrifiable nature of this ingredient; a piece of knowledge the more probable, if, as there seems little reason to doubt, they were acquainted with the art of making iron, an art which we need not deny to them when it is known to many of the inhabitants of Africa who are in a very low state of civilization.

Such are the consequences I would try to deduce from the mineralogical considerations belonging to this question.

It is now proper to examine the changes which the several substances have undergone, that we may, if possible, form some rational conjecture on the degree of heat to which they have been subjected, and on the probable means by which that heat was produced.

Where the quartz has been most exposed to the fire it has become brittle and easily pulverizable. The granite too is brittle and crumbles to pieces. The gneiss and mica-slate are also rotten where they have contained much iron, in consequence of the oxidation of that metal. Where purer they have remained unchanged, as we might expect from the well known properties of some of the varieties of mica-slate. Often where their flat surfaces have been in contact they are agglutinated from the superficial vitrification of the quartz which they contain when united to the potash produced by the fuel. This is also the cause of the glazed surface which covers the clay-slate and which has frequently occasioned numerous small pieces to adhere in one lump. In many places the mica-slate has been so softened by