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Nezv York State Museum 



series, measured perpendicular to the bedding, is known 

 as the stratigraphic interval. This interval, however, 

 varies as beds or strata thicken or thin in various direc- 

 tions due to different conditions of deposition. Some- 

 times a stratum continues to thin out in a certain 

 direction until it disappears altogether and the beds imme- 

 diately above and below come in contact. This is known 

 as pinching or lensing out of strata (figure 2). This is 



Figure 2 Cross section illustrating pinching or lensing out of 

 strata 



why a layer or bed of rock in a certain region may be 

 present in one locality but not in another. Small lenses 

 are to be looked for in rock strata. Hollows or channels 

 in gravel deposits may be filled with sand. This, in coarse 

 rocks, forms lenticular beds of sandstone which thin out 

 laterally. Some lenses of this kind are due to buried sand 

 bars. - The reverse is also found where a stronger current 

 has deposited gravel in a channel scoured out of sand. 

 Strata vary laterally in texture and porosity as well as in 

 thickness of the bed. Sandstones may become finer and 

 grade laterally into shales, and so on. The porosity of a 

 rock is the proportion of pore space to the entire volume of 

 the rock and is measured by the amount of water a stated 

 volume of rock will contain. If the spaces between the 

 grains of a sandstone gradually are filled laterally with 

 an increasing amount of cement or more and more fine 

 clayey material the porosity of the rock is gradually less- 



