NOTE XIX ... FLINT. 1. _Siliceous rocks_ from morasses; their cements. 2. _Siliceous trees_; coloured by iron or manganese; Peak-diamonds; Bristol-stones; flint in form of calcareous spar; has been fluid without much heat; obtained from powdered quartz and fluor-acid by Bergman and by Achard. 3. _Agates and onyxes_ found in sand-rocks; of vegetable origin; have been in complete fusion; their concentric coloured circles not from superinduction but from congelation; experiment of freezing a solution of blue vitriol; iron and manganese repelled in spheres as the nodule of flint cooled; circular stains of marl in salt-mines; some flint nodules resemble knots of wood or roots. 4. _Sand of the sea_; its acid from morasses; its base from shells. 5. _Chert or petrosilex_ stratified in cooling; their colour and their acid from sea-animals; labradore-stone from mother- pearl. 6. _Flints in chalk-beds_; their form, colour, and acid, from the flesh of sea-animals; some are hollow and lined with crystals; contain iron; not produced by injection from without; coralloids converted to flint; French-millstones; flints sometimes found in solid strata. 7. _Angles of sand_ destroyed by attrition and solution in steam; siliceous breccia cemented by solution in red-hot water. 8. _Basaltes and granites_ are antient lavas; basaltes raised by its congelation not by subterraneous fire. NOTE XX ... CLAY. Fire and water two great agents; stratification from precipitation; many stratified materials not soluble in water. 1. Stratification of lava from successive accumulation. 2. Stratifications of limestone from the different periods of time in which the shells were deposited. 3. Stratifications of coal, and clay, and sandstone, and iron-ores, not from currents of water, but from the production of morass-beds at different periods of time; morass-beds become ignited; their bitumen and sulphur is sublimed; the clay, lime, and iron remain; whence sand, marle, coal, white clay in valleys, and gravel-beds, and some ochres, and some calcareous depositions owing to alluviation; clay from decomposed granite; from the lava of Vesuvius; from vitreous lavas. NOTE XXI ... ENAMELS. Rose-colour and purple from gold; precipitates of gold by alcaline salt preferable to those by tin; aurum fulminans long ground; tender colours from gold or iron not dissolved but suspended in the glass; cobalts; calces of cobalt and copper require a strong fire; Ka-o-lin and Pe-tun-tse the same as our own materials. NOTE XXII ... PORTLAND VASE.