LIMESTONES. 91 



expresses the facts in the case. The magnesium carbonate present, 

 whatever its amount, simply serves to replace an equivalent amount 

 of calcium carbonate, and the resulting rock, whether little or much 

 magnesia is present, is still a pure carbonate rock. With the impuri- 

 ties to be discussed in later paragraphs, however, this is not the case. 

 Silica, alumina, iron, sulphur, alkalies, etc., when present are actual 

 impurities, not merely chemical replacements of part of the calcium 

 carbonate. 



The presence of silica, alumina, iron, and other impurities. If a 

 number of limestone analyses be examined, it will be found that the 

 principal impurities present are silica, alumina, iron oxide, sulphur, and 

 alkalies. 



Silica when present in a marble or crystalline limestone is usually 

 combined with alumina, iron, lime, or magnesia, and occurs therefore 

 in the form of a silicate mineral. In an ordinary limestone it is very 

 often present as masses or nodules of chert or flint, or else combined 

 with alumina as clayey matter. In the softer limestones, such as the 

 chalks and marls, the silica may be present as grains of sand. 



Alumina is commonly present combined with silica either as grains 

 of a silicate mineral or as clayey matter. 



Iron may be present as carbonate, as oxide, or in the sulphide form 

 as the mineral pyrite. 



Sulphur is commonly present in small percentages in one of two 

 forms: as pyrite or iron disulphide (FeS 2 ) or as gypsum or lime sul- 

 phate (CaS0 4 + 2H 2 0). 



The alkalies soda and potash are frequently present in small quan- 

 tity, probably in the form of carbonates. 



Geologic and geographic distribution of limestones. Limestones 

 occur in every state and territory in the United States, though of course 

 some states (Delaware, North Dakota, Louisiana, etc.) are so poorly 

 supplied that they can never become important lime producers, while 

 other states are almost entirely underlain by limestone strata. Geo- 

 logically, the limestone utilized in various parts of the United States 

 ranges entirely through the geological column, from the pre-Cambrian 

 to the Pleistocene, inclusive. 



Under such conditions of wide geographic and geologic distribu- 

 tion it is not practicable to give a summary of any value in the present 

 volume. The list of references given in the following pages will enable 

 the reader to ascertain the facts regarding the limestones of any given 

 state in which he may be interested. 



