Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/428

 The organic bodies that afforded nuclei to these nascent flints, appear to have been dispersed pretty uniformly through the original compound mass which is now divided into beds of chalk and flints, but it is not easy to determine what cause it was that regulated the distances at which the beds of flint have been disposed, or to say why we sometimes find organic bodies preserved in flint, at other times enveloped and filled only by pure chalk. The solution of the latter question may be, that different genera of organic remains afforded centers that attracted the silex with unequal force, and that this will in some degree explain the phenomenon so common in the chalk formation, that bodies allied to the genus spunge and alcyonium, are most frequently preserved in flint and chalcedony, whilst shells and other bodies, which in their natural state were more calcareous, generally have their form retained by chalk or calcareous spar.

In cases of many of these silicified spunges and alcyonia (of which there is in the Ashmolean Museum at Oxford, an extensive collection, from Henley and Stokenchurch, in Oxfordshire) the outer crust being composed of flint in its common state, represents rudely the outline of the body inclosed. But the internal structure retains traces of all its tubes and fibres, most delicately preserved in