icS NATURE OF LIMESTONES NOW FORMING. 



exposed to the sun at low water, and thus hardened by chemical 'preci- 

 pitation. Hence coral limestone is far from being formed by corals 

 only, and at some depth the cavities in the corals themselves are 

 always obliterated by carbonate of lime deposited in them by infiltrat- 

 ing waters. 



Remarkable calcareous deposits of small extent sometimes occur on 

 land. Dr. Phene l has described a deposit of carbonate of lime which 

 has buried up the Roman city of Hierapolis in Anatolia, and covered 

 a large extent of country. Under the most eccentric and beautiful 

 forms half the city is submerged by a mass of intensely hard rock 

 which blocks up streets, temples, and arches. After reaching the 

 level of its source it ran over the natural aqueducts which it formed 

 as it went, only to begin and build up newer ones from a lower level. 

 Six or eight of these walls occur, each nearly fifty feet in height, show- 

 ing that many hundred feet of deposit have taken place since the 

 Roman occupation. Part of the rock is perfectly white, having the 

 aspect of drifted snow and frozen cascades. Other parts of it are as 

 perf sctly black. This is an example of the way in which springs such 

 as have formed the deposits of travertine in many parts of Italy and 

 the Auvergne may contribute to form limestones on land ; while if the 



Fig. 42. Deposit of Trave-tin at a Cascade. 



spring had flowed into a lake the deposit might have been more evenly 

 spread, but would probably not have been less in amount. 



On the shores of the Bahamas, the rocks are a hardened deposit of 

 fine calcareous mud, but in some can be detected fragments of coral- 

 lines, corals, mollusca, and foraminifera. In all seas the mollusca 

 contribute largely to the formation of limestones, not merely by the 

 accumulation of shell-beds but by becoming disintegrated and broken 

 up a condition which may result either from the mechanical action 

 of currents or from decomposition when the organic substance which 

 bound their particles together is removed. 



Some shells like Pinna and Inoceramus are chiefly composed of 

 minute prisms, which may be set free as fibres when the organic matter 

 of the shell is lost. Similarly encrinites and all echinoderms fall to 



1 Brit. Assoc. Reports, 1879, Sect., p. 344. 



