4 8 GEOLOGY OF THE ISLE OF WIGHT 



The Chalk Sea evidently abounded in siliceous organ- 

 isms, and it cannot be doubted that it is from such 

 organisms that the silica was derived, which has formed 

 the masses of flint. Silica occurs in two forms in a 

 crystalline form as quartz or rock crystal, and as amor- 

 phous, i.e., formless or uncrystalline (also called opaline) 

 silica. The siliceous skeletons of marine organisms are 

 formed of amorphous silica. Flint consists of innumer- 

 able fine crystalline grains, closely packed together. 

 Amorphous silica is less stable than crystalline, and is 

 capable of being dissolved in alkaline water, i.e., water 

 containing carbonate of sodium or potassium in solution. 

 If the silica so dissolved be deposited again, it is generally 

 in the crystalline form. It seems probable, therefore, 

 that the amorphous silica of the skeletal parts of marine 

 organisms has been dissolved by alkaline water percolating 

 through the strata, and re-deposited as flint. 



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, 



