98 



The sedimentary origin is clearly marked in both chalk and 

 limestone and this gives a characteristic appearance of strata in 

 the calcium rocks which is typical of itself alone. This horizontal 

 bedding, however, is not indefinitely maintained. Volcanic action 

 and earth movements have produced foldings and crumplings ia 

 the chalk whereby the strata have been tilted to every degree of 

 obliquity. The Ballard Down fault is an excellent illustration of 

 this. 



The history of chalk definitely belongs to the study of geology, 

 but for purposes of geography it is well to bear in mind the variety 

 of chalk due to marine infusoria and that due to innumerable 

 corallines where no trace of organism can be found. There is yet 

 a metamorphic form common in limestone areas in the nature of 

 stalactites and stalagmites. These are entirely due to the solu- 

 bility of chalk and consequent evaporation of the solvent whereby 

 the soluble constituent is deposited in a variety of icicle-like forms, 

 such as is to be seen in the caves of Cheddar in the Mendips. 

 Occasionally the crystalline form of calcium carbonate (calcite) 

 results and the chalk cliff itself may be richly impregnated with 

 crystals of iceland spar or calcite. Much calcite of this descrip- 

 tion exists in the Purbeck limestone and chalk. The formation of 

 this variety of crystal is evidently assisted by earth crumpling and 

 by internal heat. 



In this connection, viz., of chemically formed calcium rocks, one 

 must scarcely forget the feature of calcium strata known as 

 "petrifying springs" common at Matlock and other places. 

 Rich deposits of chalk are laid upon objects subjected to the in- 

 fluence of the calcium-charged waters. Many of the chalky casts- 

 of bivalves found at Studland are the result of similar continued 

 action and the fossil forest of Lulworth has almost identical forma- 

 tion. The volcanic areas are not without the geographical interest 

 associated with calcium. Hot springs and geysers abound, con- 

 taining carbonate of calcium, and the water of these passing over 

 large areas of land covers the soil richly with beds of Ci tufa " or 

 travertine so familiar a scene in the landscape of Iceland, Rome, 

 and Naples. 



The coral polype or zoophyte is also responsible for other 

 unique calcium formations arbitrarily graded as " fringing reefs,'* 

 "barrier reefs," and " atolls." Our own locality, and in fact our 

 own islands, are almost free from present actual coral formations. 



Coral reefs produce specialised geographical features and are- 

 peculiarly isolated patterns of land formations consisting almost 

 entirely of carbonate of calcium derived from the coral polype. 

 The coral polvpe belongs to the same class of organisms as the 

 sea anemone, the gelatinous, flower-like creature common in rock 

 pools around the coast. They extract the calcium carbonate from 

 sea water and deposit it in a cylindrical body. These polypes 

 multiply by budding, in this way the colony increases. As the 

 polypes die their soft portions decay, whilst the hard calcareous- 

 skeletons remain to form coral rocks. The creature is extremely 



