PETROLEUM AND NATURAL GAS IN NEW YORK 421 



There are still other strata in the column which may contain 

 water, oil or gas. Dolomitic limestones that have had a particu- 

 lar history are sometimes as porous as the coarsest sandstone. 

 True limestones also in some of their phases show considerable 

 storage capacity for gas and occasionally for water. 



Porous dolomites seem to have resulted from the replacement 

 of their rock substance. The first stage is pure limestone. Then 

 by some change in the character of the sea water, magnesia is 

 supplied in such quantity that it replaces one half of the lime- 

 stone atoms, transforming the rock into a true dolomite. But 

 the atom of magnesian carbonate is demonstrably smaller than 

 the atom of calcium carbonate which it has replaced and if the 

 original rock volume is maintained, vacant spaces must be left 

 in the mass. Not all dolomites are porous. It is conceivable 

 that dolomites originating in a different manner from that men- 

 tioned above may be compact and close grained rocks. 



Concerning the reservoir qualities of limestones, we have not 

 as good means of judging as of the rocks already described in 

 which the porosity is a natural consequence of their physical 

 state. Where limestones act as reservoirs there must be vacant 

 spaces between the layers or beds of the stratum. That they are 

 not true reservoir rocks is evident from the fact that they do 

 not as a rule contain fresh or salt water in large volume. It 

 seems probable, however, that the structural features of the 

 strata contribute to the storage of gas. 



