5. Clay from trap. At the foot of the softer rocks of trap-formation, such as wakke, clay-porphyry, and some varie ties of grunstein and hornblende rock, are found in beds of clay, evidently ori ginating from the gradual disintegration of these by the weather.
6. Marly clay. The colour of this is bluish or brownish red : it occurs either compact or foliated : it has a soft unc tuous feel, takes a polish by friction with -the nail, is very plastic, more or less gritty, though not so much so as the com. mon alluvial clay. It burns to a brick of a buff or deep cream colour, and at a high heat readily enters into fusion. It effervesces strongly with acids, and con. tains from one-fourth to one-tenth of carbonated lime. It originates some times from the decomposition of com pact argillaceous lime-stone ; but more frequently from the softer slaty varie ties usually called stone-marl. It is largely employed as a manure, and where the calcareous part does not exceed 10 or 12 per cent. it is esteemed as a mate rial for bricks.
7. Clay from metallic veins. Its colour is grey, verging into bluish, greenish, and yellowish, or red. It has &smooth unctuous feel ; is very tenacious ; often contains sulphuric acid, and certain me tallic oxides, which are never observed in other clays, such as lead, silver, anti mony, copper and bismuth. is found in metallic veins.
8. Alluvial clay. The circumstances
which characterize alluvial clay are the following. It contains a larger propbr tion of quartz sand than the preceding ; rounded pebbles of various kinds are also imbedded in it ; thus showing it to have been carried from its native situa tion, and mingled in its progress with a variety of extraneous bodies. At least three kinds of it may be distinguished; viz. pipe clay, potter's clay, and chalky clay. Pipe clay is of a greyish or yellow ish white colour, an earthy fracture, and a smooth greasy feel ; it adheres pretty strongly to the tongue ; is very plastic and tenacious ; when burnt, is of a milk white colour ; is difficultly fusible, though much more so than porcelain clay, from which it is further distinguished by its superior plasticity, and the sand it contains. It is manufactured into tobac co pipes, and is the basis of the white or queen's-ware pottery. Potter's clay is of a reddish, bluish, or greenish colour ; has a somewhat fine earthy fracture, and a soft, often greasy feel : it adheres to the tongue, and is very plastic. It burns to a hard, porous, red brick ; and in a higher heat runs into a dark colour ed flag. When tempered with water, and mixed with sand, it is manufactured into bricks : those varieties that are the most free from pebbles are made into tiles, and coarse red pottery. See ALu