440 W. F. PROUTY CRYSTALLINE MARBLES OF ALABAMA 



difference of the topographic situation of the marbles of Vermont as com- 

 pared with Georgia and Tennessee is apparently in part due to the greater 

 chemical activity in weathering in warmer climates, but in the case of 

 the Alabama deposits this explanation is not alone sufficient. A compara- 

 tive study of the cross-sections through the different deposits does, how- 

 ever, supply the needed information. 



The Alabama marbles, figure 3, are separated from the overlying schist 

 by a fault which provides a zone of more rapid solution than is the case 

 in Georgia, where the climatic conditions are similar. 



Throughout the marble field there are very few natural exposures of 

 marble, and these occur only along the streams or where the rocks have 

 been sharply folded or faulted. The fact that the marble in some of the 

 earlier, developed quarries proved very unsound is clue to their location 

 on these natural outcrops above the drainage levels, where fracture, due 

 to the unusual disturbance, is greater than elsewhere and where weather- 

 ing, due to the exposed position of the rock, has been abnormally great. 



STRUCTURAL CONDITIONS 



Faulting. — The structural conditions in the marble area are such as 

 would result from a shortening of the earth's crust, due to lateral pres- 

 sure, with accompanying reverse faulting, minor folding, and shearing- 

 stresses. The reverse fault on the southeast side of the marble area 

 varies considerably in throw. In many places the main fault is paral- 

 leled by a second fault a few hundred feet distant to the northwest in the 

 marble valley. 



The general northeast and southwest trend of the marble valley- is 

 sharply altered in several places by offsets (see map of the marble de- 

 posits). The largest offset has a lateral displacement of the valley of 

 over three miles and is caused by a combined fault and fold. In many 

 places small offsets occur along nearly parallel oblique or dip faults. This 

 is well illustrated in the accompanying figure (figure 4), which shows the 

 condition near the Herd quarry, a little north of Sylacauga. 



Dip. — The dip of the marble is remarkably uniform throughout the 

 field. It varies, as a rule, but little from 30 degrees and is in a general 

 easterly direction. 



Schist osity. — In most of the marble exposures there is evidence of 

 shearing stresses. Slipping has taken place for the most part along the 

 schist planes, but in some cases the slips are evident in the marble itself, 

 forming the so-called "reeds" of the quarrymen. This slipping in the 

 marble is sufficient in one locality to render the marble distinctly schis- 

 tose. Figure 5 shows a hand specimen of such marble taken from the 



