CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 2J 



a small number of scattered grains of quartz. In other cases more 

 or less silicate of alumina is present, and the rock becomes an argil- 

 laceous limestone, passing, in extreme cases, into a " calcareous 

 shale." In other cases, again, the limestone may be more or less 

 highly charged with minute particles of carbon, or may be more or 

 less impregnated with certain hydrocarbons, the rock becoming a 

 carbo7iaceous limestone or a bituminous limestone, as the case may be. 

 The term " marble " is one of no very precise signification, any 

 limestone which is sufficiently hard and compact to take a high 

 polish being usually spoken of under this name. " Chalk," again, 

 from a purely lithological point of view, is a soft pulverulent lime- 

 stone, but it is occasionally quite hard and compact ; and its truly 

 essential characters depend upon its organic structure, which will be 

 more fully considered later on. 



(2.) Chemical Constitution. — The most important variation in 

 limestones, from a chemical point of view, is established by the 

 presence in the rock of more or less carbonate of magnesia. The 

 presence of a certain amount of magnesia in a limestone is a very 

 common phenomenon, and often only admits of detection by means 

 of chemical analysis. Limestones which contain a variable and 

 comparatively small amount of carbonate of magnesia are spoken of 

 as " magnesian limestones," and they often differ little or not at all 

 from ordinary limestones in either appearance or structure. Where 

 there is a notable proportion of carbonate of magnesia present, the 

 limestone often assumes a brownish or yellowish colour, with a 

 sandy aspect, while it shows a marked tendency to undergo 

 secondary crystallisation. This is shown in some cases by the 

 development of a concretionary structure in the rock, the so-called 

 " concretions " being truly the result of an imperfect form of crystal- 

 lisation. Thin sections, also, of such magnesian limestones invari- 

 ably show that the rock has undergone more or less extensive re- 

 crystallisation, subsequent to consolidation ; and the organic fragments 

 originally present in the rock have been thereby more or less largely 

 obscured, or, it may be, completely obliterated. Owing, further, to 

 the comparative insolubility in water of carbonate of magnesia as 

 compared with carbonate of lime, the larger calcareous organisms 

 (such as the shells of Molluscs) in the more highly magnesian lime- 

 stones have been commonly dissolved out of the rock, and are now 

 only represented by casts and moulds. Where the carbonate of 

 magnesia is present in a limestone in such quantity as to form with 

 the carbonate of lime a true double carbonate, the rock is what is 

 strictly called a " dolomite." The true dolomites, when examined 

 microscopically, are always found to be more or less intensely crys- 

 talline. In some cases the recrystallisation to which the rock has 

 been subjected subsequent to its original formation has not been 



