l6 INTRODUCTION. 



In all cases, the carbonate of lime which exists in a limestone has 

 previously existed in solution in water, either in the water of a spring, 

 river, or lake, or in that of the ocean itself. Owing, in fact, to the 

 ready solubility of calcium carbonate in water holding in solution 

 a certain proportion of carbon dioxide, a larger or smaller quantity 

 of this mineral is invariably found dissolved in all natural waters, 

 whether fresh or salt, since these waters are always to some extent 

 charged with this solvent gas. There are two principal methods by 

 which the carbonate of lime held in solution in water may again 

 assume the solid form. One of these methods consists in the chem- 

 ical precipitation of the carbonate of lime from the water. This 

 takes place whenever the carbonic acid in the water becomes so far 

 reduced in quantity that it is no longer able to retain in solution all 

 the lime that had been previously dissolved ; or whenever the water 

 undergoes partial or complete evaporation ; or, again, when water 

 which had been enabled by a high temperature to take up an excess 

 of lime, is subjected to cooling. Various well-known calcareous 

 deposits, such as the " stalactites " and " stalagmites " of limestone 

 caves, and the " calcareous tufa " and " travertine " of springs, are 

 produced in this way by the direct precipitation of carbonate of lime 

 from solution. All limestones deposited in this chemical way directly 

 from saturated solutions are necessarily composed of larger or smaller 

 crystals of carbonate of lime, and the microscope will show that their 

 structure is more or less clearly crystalli?ie. They may contain the 

 remains of animals or plants, as is not uncommon in the spongy 

 calcareous tufa deposited by " petrifying springs " ; but as such 

 remains are usually only encrusted by the precipitated lime, and are 

 not infiltrated, they generally become dissolved out in the course of 

 time, leaving cavities which mark their former presence. Calcareous 

 deposits formed by direct precipitation occasionally occur on a large 

 scale, and thus become geologically important, but the ordinary 

 limestones are formed in a different way, and are of much greater 

 palaeontological interest. 



By far the most general method in which the dissolved carbonate 

 of lime in water may be converted into the solid form is by the vital 

 chemistry of animals and plants. Very many animals, and a con- 

 siderable number of plants, have the power of abstracting from the 

 water the carbonate of lime which it holds in solution, and of build- 

 ing up in this way a calcareous skeleton. Hence, while the waters 

 which percolate through the earth's crust are constantly taking up 

 fresh carbonate of lime, this is being as constantly removed from the 

 waters of rivers, lakes, and the sea, and again converted into the solid 

 form, by the agency of living beings. It is owing to the fact that 

 animal life is much more abundant in the sea than in rivers and 

 lakes, that sea-water contains a proportionately smaller proportion of 



