which is not so evident, as they differ in many respects from the chalk flints in their usual state. Of these some are of a deep red colour with a great degree of transparency resembling carnelian: others are of a yellow chalcedony often translucent and even botryoidal, and they pass into a kind that is yellow, opake, and of a waxy lustre. Others again appear to be nearly allied to hornstone, and are frequently of irregular shapes, which are probably those of the original nodules.
Another remarkable class of siliceous pebbles is found either mixed with all those above mentioned, or alone, or cemented together into a pending-stone. These appear to have been originally formed of concentric coats or layers of different colours, which vary in almost every specimen. The colours are for the most part yellow, brown, red, bluish-black, grey, and white: but these run into each other by an infinite number of shades. Others are spotted or clouded with different tints, and have much the appearance of Egyptian pebbles. They take an excellent polish, and are then often extremely beautiful.
These last appear rather more to resemble agates than chalk flints. They are never found of a large size, seldom exceeding two inches in diameter, and generally are not more than one inch: they are of an oval or flattened form, which appears to have been their original figure, although they have evidently been subjected to a certain degree of attrition.
All the above mentioned pebbles are sometimes surrounded by crusts; and it does not appear clear whether these are not sometimes original, though perhaps stained by the ochreous substances in which they have been imbedded, or whether they are generally the effect of decomposition. Flint appears to be one of the most unchangeable substances with which we are acquainted. We see
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