STRUCTURAL CONDITIONS 



445 



to weathering, than it is deeper in the deposit. The chief agents of 

 unsoundness in the marble in the upper few feet of the quarry are the 



w 



Figure 8. — -Photomicrograph of the commercial Oracle of Alabama crystalline Marble 



magnified fifty Diameters 



Note the interlocking character of the crystals and the twinning bands. The thin 

 section from which this photograph was taken came from about one-half inch below the 

 surface of a slab of marble which has been exposed to continuous weathering for over 

 60 years and which shows practically no iron stain and which has retained the very 

 finest lettering in an almost perfect condition. The average maximum grain size is .57 

 mm. This makes the marble a little finer grained than the average Vermont marble. 



so-called "slicks" (vertical planes of parting), which run directly down 

 the dip and decrease with depth. These planes of weakness are similar 

 to the vertical joints which occur 

 in concrete dams at right angles to 

 their length, which result from ex- 

 pansion and contraction due to 

 changes of temperature. This ten- 

 dency for the marble to be less UU- Figure 9.— Diagram of Schist Lines in Block 



sound deeper in the quarry is also of MarMe at variance with general bed- 



1 ... ding Direction, due, it is thought, to dif- 



found to be the case in the indi- ferential Movements in the layer because 



vidual beds. Figure 10 represents of Brag ' 



the decrease in throw and the final disappearance of the small faults seen 



in one of the blocks taken from the quarry. 



XXXII — Bull. Geol. Soc. Am., Vol. 27, 1915 



Schist 



