LIME AND CEMENT INDUSTRIES 643 



G. P. Merrill^ states that " it is very probable that few of our 

 limestones are wholly from organic remains, but are in part at 

 least chemical deposits. The alternation of the beds of snow 

 white, blue gray, greenish and almost black layers, as in the Ver- 

 mont quarries, may be best explained perhaps on the assumption 

 that the white layers resulted as deposits from solution, while 

 the darker layers are but beds of indurated shell mud and sand 

 colored by the organic impurities they contained at the time they 

 were first laid down/' 



Fossils are sometimes plainly apparent in the limestone, but 

 very often the shells become comminuted before settling on the 

 ocean bottom, or they may be broken by the pressure of other 

 material deposited on them, so that not infrequently limestones 

 show no trace whatever of organic remains. Limestones of great 

 purity have generally been deposited in the deeper parts of the 

 ocean, or at least far enough away from the shore to prevent 

 their contamination by silicious or argillaceous sediments brought 

 down to the sea by rivers. The varying intermixture of such 

 classes of material with the calcareous mud results in the forma- 

 tion of all grades of rock between a limestone and sandstone on 

 one hand, and a shale on the other. A silicious limestone is one 

 with silicious impurity, while a mixture in which the silica pre- 

 dominates is called a calcareous sandstone. In the same w:ay, 

 we may have a shaly or argillaceous limestone or a calcareous 

 shale. 



The consolidation of the limestone particles may be due to the 

 precipitation around them of lime carbonate from the sea water, 

 or it may be due to the percolation of carbonated meteoric waters 

 through a mass of calcareous sand. 



CHEMICAL COMPOSITION 



Pure limestone is composed of carbonate of lime or the mineral 

 calcite and consists of 56^ of oxid of lime and 44^ of carbon 



*- , I, 



1 Stones for building and decoration. 1891. p. 79. 



