650 ]N"E\V TOEK STATE MUSEUM 



facilitates the extraction of the stone, jet they also serve as a 

 ready means of entrance for the weathering agents. 



Where the topography of the country is such that outcrops are 

 scarce, the character of the vegetation may often serve as a clue 

 to the character of the underlying rock. In 'New York the sur- 

 face is usually covered by glacial drift and hence the bed rock 

 exerts no influence on the tree growth, as it does in the southern 

 states where residual soils are common. Streams which disap- 

 pear in caves, and calcareous springs may also be looked on as evi- 

 dencing the presence of lime rock. 



In searching for calcareous beds, if the rocks are steeply tilted, 

 it is better to follow a line at right angles to the strike, thus pass- 

 ing over the edges of the different upturned beds. In regions 

 of little or no dip this plan is valueless, and a careful inspection 

 must be made of ravines, valleys, and railroad cuttings. If the 

 beds dip, the apparent thickness of the rock bed at the surface, 

 measured at right angles to the strike, will be much greater than 

 its real thickness, the difference being greater the less the dip. 

 A rapid means of determining the real thickness of beds whose 

 dip is under 45° is by the following rule: multiply yV of the ap- 

 parent thickness by -=- of the degree of dip. Thus, if the apparent 

 thickness were 100 feet, and the dip 15°, the actual thickness 

 would be 100 -^ 12 = 8f 8i x 3 = 25 feet. 



COLOE OF LIMESTONES 



An absolutely pure limestone would be white, that being the 

 natural color of calcite, but most limestones are colored either 

 by iron oxid or organic matter. The former gives yellow, brown, 

 red or gray colors, depending on the form of combination and 

 stage of oxidation; Avhile organic matter colors the limestone 

 gray to black. A very small percentage of organic matter may 

 color a limestone black, the black limestone of Fairhaven con- 

 taining, for instance, less than 1^ of impurities. 



