THE MATERIALS OF STRATA. 45 



occurs in irregular masses. Both iron and sulphur 

 may have been liberated in the decay of marine 

 plants. As the iron pyrites decomposes in con- 

 tact with the air, its sulphur is converted into an 

 acid, which dissolves the substance oi shells, ex- 

 pelling the carbonic acid, and forming a hydrated 

 sulphate iA lime, known as selenite. Occasion- 

 ally phosphate of lime form concretions in clays 

 and mineralizes fossils. (Mays are commonly 

 formed in deeper water than sands, and further 

 from the shores which furnished the sediment. 



Limestone, 



T.im - differ from other water-formed 



rocks in not being sediments. Their particles 

 have grown, as portions of organisms; and have 

 become rock substance, when the animals or 



plants died, which separated the carbonate of 



lime from water. Sometimes limestone is pre- 

 cipitated by evaporation of water. The carbonate 



of lime which forms limestones is usually in the 

 mineral condition of calcite. 



beds <>f limestone maybe deposited over the 

 whole sea-bed, whether the water is shallow or 

 deep. As a rule they are most noticeable in the 

 open ocean, beyond the limits to which sediments 

 are carried. Limestone may be formed near into 

 shore; and when the rock is dissolved away by 

 acids, in some cases nothing remains but a vary- 

 ing percentage of siliceous sand. Rocks of that 

 kind are termed calcareous grits. 



Evidences of the shallow-water origin of some 

 oolitic limestones in the west of England, are also 

 seen in current bedding, which characterizes some 

 oolitic rocks, and is as marked as in sandstones. 



