Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/208

 The lowest sandstone, called the little flint, is the eighty-fifth in number, and is about fifteen feet thick; the lower part of it is very coarse, and full of pebbles of quartz; the upper is of a finer grain, and sometimes is rendered very dense and hard by an intimate mixture of iron ore; it occurs at the depth of seven hundred and live feet. Vegetable impressions are met with in most of the sandstone beds, but I have not heard of their containing any shells.

The clay porphyry occurs only once in the whole series; it forms a bed nine inches thick, at the depth of seventy-three feet from the surface. The basis of this rock is a highly indurated clay of a liver-brown colour, in which are imbedded grains of quartz, of hornblende and of felspar.

The indurated clay is mostly of a bluish-brown colour with a tinge of olive, which by decomposition passes to bluish-grey and ash-grey, and, when containing much iron, to ochre yellow and red, and becomes very tough and plastic. In some beds it is compact, dull and smooth, but somewhat meagre to the touch, and is then usually distinguished by the name of clod; in others it is glossy, unctuous, and tending to a slaty texture, and is then called clunch. It incloses subordinate beds of clay ironstone in the form of balls more or less compressed, or in flat pieces of considerable magnitude, and two or three inches thick. Besides vegetable impressions, it contains a few shells; small mytili in particular are found in the iron ore, called crawstone, which lies immediately above the little flint. One of the most remarkable beds of this clay is called the pinny' or penny-measure. It is the sixty-third in number, lying the next below the big flint, and occurs at the depth of five hundred and eighty feet. Its thickness in Madeley colliery is scarcely seven feet, but at Ketley is full twenty-seven feet; in the latter district it contains subordinate beds of ironstone in flattened nodules, called pennystone,