As the silica was deposited, chalk was removed. The large irregular masses of flint lying in the Chalk strata have clearly taken the place of chalk which has been removed. Water charged with silica soaking through the strata has deposited silica, and at the same time dissolved out so much carbonate of lime. Bivalve shells, originally carbonate of lime, are often replaced, and filled up by flint, and casts of sea urchins in solid flint are common, and often beautiful fossils. This process of change took place after the foraminiferal ooze had been compacted into chalk strata; and to some extent at any rate, there has been deposition of silica after the chalk had become hard and solid; for we find flat sheets, called tabular flint, lying along the strata, or filling cracks cutting through the strata at right angles. But in all probability the re-arrangement of the constituents of the strata took place in the main during the first consolidation, as the strata rose above the sea-level, and the sea-water drained out. A suggestion has been made by R. E. Liesegang, of Dresden, to explain the occurrence of the flints in the bands with clear interspaces between, which are such a marked feature of the Upper Chalk. He has shown how "a solution diffusing outward and encountering something with which it reacts and forms a precipitate, moves on into this medium until a concentration sufficient to cause precipitation of the particular salt occurs. A zone of precipitation is thus formed, through which the first solution penetrates until the conditions are repeated, and a second zone of precipitate is thrown down. Zone after zone may thus arise as diffusion goes on." He suggests that the zones of flint may be similar phenomena, water diffusing through the masses of chalk taking up silica till such concentration is reached that precipitation takes place, the water then percolating further and repeating the process.[8] The precipitation of silica and replacement of the chalk occurs irregularly along the zone of precipitation, forming great irregular masses of flint, which enclose the sponges and other marine organisms that lay in the chalk strata. Where a deposit of silica has begun, it will probably have determined the precipitation of more silica, in the manner constantly seen in chemical precipitation; and it would seem that siliceous organisms as sponges have to some extent served as centres around which silica has been precipitated, for flints are very commonly found, having the evident external form of sponges.