MAGNESIAN LIMESTONE 



213 



FIG. 76. — Chalk from Kansas. X 45. 

 (Drawn from a photograph by the Geo- 

 logical Survey of Iowa.) 



composed of the shells of Foraminifera, and closely resembles the 

 foraminiferal oozes forming to-day at the bottom of the sea 

 (p. 1 76) . A chalky deposit may, 

 however, be formed from the 

 debris of corals ground up by 

 the waves. 



Hydraulic Limestone contains 

 a considerable quantity of clay, 

 and the mortar made from it 

 has the property of setting under 

 water, so that it is used in the 

 manufacture of hydraulic cement. 



The ordinary massive marine 

 limestones are named from the 

 character of the organic ma- 

 terial which predominates in them. Thus, we have coral lime- 

 stone, foraminiferal limestone, made up of the shells of very large 

 extinct forms of the Foraminifera (Fusulina, Nummulites, Orbito- 

 lites, etc.), crinoidal limestone, shell limestone, and the like. 



Though much the larger part of the limestones is of animal origin, 

 yet certain seaweeds contribute extensively to formation of these 

 rocks, and there is much reason to believe that chemical precipita- 

 tion is of greater or less importance in nearly all varieties of the rock. 



Dolomite, or Magnesian Limestone, is a compact, granular rock 

 of white, grey, or yellow colour, composed of the carbonates of 

 lime and magnesia. Nearly all limestones contain some carbonate 

 of magnesia, but the name dolomite is given only to those with a 

 considerable percentage of that substance (5 to 20%). How far 

 this rock is made up of the mineral dolomite, and how far it is 

 merely a mixture of the two carbonates, is uncertain, as is also the 

 way in which the rock was formed. Dolomite contains a much 

 larger proportion of magnesia than the shells or tests of any known 

 animals, and this ingredient must therefore have been added after 

 the accumulation of the calcareous organisms. Opinions differ as 

 to just how this has been accomplished, but probably the magnesia 

 has been derived from the strong brine of lagoons and salt lakes. 



