16 Nezv York State Museum 



Sedimentary Rocks 



Origin of materials. Sedimentary rocks may have 

 one of three origins, mechanical, chemical or organic. 

 Those of a mechanical origin, the clastic rocks (Greek 

 klastos, broken), are derived from sediments deposited 

 through the agency of water and the air and here belong 

 conglomerates and breccias, sandstones, shales and lime- 

 stones, the rocks which are of most importance to us here 

 and which will be discussed in greater detail below. 



The great bulk of our limestones are of organic and, 

 secondarily, of mechanical origin, but some are chemical 

 precipitates. Calcareous tufa is one of the most exten- 

 sive of these. It is formed by springs that issue in lime- 

 stone regions and deposit the excess of lime that they hold 

 in solution. Calcareous tufa is mostly porous and not 

 infrequently incrusts plants and other objects, but there 

 are also massive deposits forming the so-called "Mexican 

 Onyx." Other carbonate of lime precipitates of a com- 

 pact character are the stalactites and stalagmites of caves. 

 Sometimes beds of limestone are composed of large, 

 rounded concretions formed by the deposition of carbon- 

 ate of lime, generally around a nucleus ; or again such 

 concretions may form distinct layers in shales as is seen 

 in places in our Marcellus and Portage beds. Dolomite 

 which is a carbonate of lime and magnesium is formed as 

 a primary deposit in cut-off basins of sea water, espe- 

 cially where there is an excess of chlorides in the water, 

 and as an original precipitate in portions of the sea where 

 the solution has become concentrated. In some cases 

 dolomitic limestones are of secondary origin. The mag- 

 nesia content of the original limestone was probably 

 mostly derived from calcareous algae, but through solu- 

 tion of the carbonate of lime by ground water the pro- 



