4 w Second Appendix. contain be much oxygenated, as it often is in thofe of a red colour, but even thefe are frer quently magnetic by reafon of the fliorls embo- died m them. ^ The component ingredients of lavas are va- rious, according to the nature of the original ltones, and the accidents they meet wich in their liquified ftate. Mr. Dolomieu found them to contain from 40 to 60 per ct. of filex, from 16 to 3 of magnefia, from 5 to x of calx, and from 6 to 25 of iron. Ponces, 184. § v. Of Volcanic Albes, Sand, Pouzzolana, Trafs, Tufa, and Piperino. ift, Of Volcanic Afhes. Their colour is brownifh or reddifh grey. 3 loofe and dufty confiftence, very light and ftibtle, and fmooth to the touch. Slowly diffufible in water, and when wet fome- what duflile. They cffervefce flightly with acids, and con- tarn about \ their weight of argiil, a fmall pro- portion ofcalx, magnelia, and iron; and the re- maipder is filiceous. In different volcanos, nay in different eruptions of the famc volcano, the properties diffcr. Mag- t netifm