84 LITHOLOGICAL GEOLOGY. 



morphic as well as the schists. Titanic iron-ore occurs in great beds 

 of like extent in Canada. (See \ 72.) 



Franklinite, an iron-zinc ore, is also one of the metamorphic rocks in northern 

 New Jersey. 



3. Calcareous Rocks. — Carbonates and Sulphates. 



90. (1.) Massive Limestone. — Uncrystalline Limestone. — Most lime- 

 stone has been formed from shells and corals ground up by the 

 action of the sea and afterwards consolidated. The colors are dull 

 gray, bluish, brownish, to black. The composition is usually the 

 same as that of calcite, carbonate of lime ($ 69), except that impu- 

 rities, as clay or sand, are often present. In texture they vary 

 from an earthy-looking limestone to a very compact semi-crystal- 

 line one ; and from this kind the passage is gradual also to the 

 true crystalline. 



(2.) Magnesian or Dolomitic Limestone ($69 [24]).— Consists of 

 carbonate of lime and magnesia, but is not distinguishable in color 

 or texture from ordinary limestone. The amount of carbonate 

 of magnesia present varies from a few per cent, to that in dolo- 

 mite. Much of the common limestone of the United States is 

 magnesian. 



Analyses of magnesian limestones : — 1. Lower magnesian (Calciferous epoch), 



by D. D. Ow<m ; 2, id. of Iowa, J. D. Whitney ; 3, Galena limestone of Iowa, 



Whitney ; 4, Niagara limestone, by Beck ; 5, Carboniferous limestone of Iowa, 



Whitney. 



1. 2. 3. 4. 5. 



<a+ rwv New Garna- Lock- Mt. Plea- 



st. oioix. Galena ville port sant> 



Carbonate of lime 48.24 52.47 52.01 75.65 57.15 



Carbonate of magnesia.... 42.43 42.13 42.25 20.70 39.24 



Carbonate of iron 1.78 0.93 



Insoluble (sand) 2.74 2.75 4.43 2.25 2.18 



Oxyd of iron and alumina 6.14 0.87* 0.38 Ox. iron 0.35 1.06 



Moisture 0.40 andloss 1.05 0.31f 



99.95 100.00 100.00 100.00 99.94 



In some limestones the fossils are magnesian limestone, while the rock is 

 common limestone. Thus, an Orthoceras in the Trenton limestone of Bytown, 

 Canada (which is not magnesian), afforded T. S. Hunt — Carbonate of lime, 56.00, 

 carbonate of magnesia, 37.80, carbonate of iron, 5.95 = 99.75. The pale-yellow 

 veins in the Italian black marble called " Egyptian marble" are dolomite, ac- 

 cording to T. S. Hunt, and a limestone at Dudswell, Canada, is similar. 



(3.) Hydraulic Limestone. — An impure or earthy limestone, con- 



* Alkalies and loss. f Carbonate of soda and trace of potash. 



