440 



GEOLOGY. 



into sand in being dug out, and is used for mortar sand much more than 

 for building stone. Some of the Hudson River shales of scarcely less 

 age are more nearly clay than hard rock. But these are examples of 

 excessive slowness and shghtness of change. In general, all but the 

 most recent deposits show notable progress in reconstruction. 



Fig. 362. — Figure showing the elongation of pebbles resulting from pressure. Car- 

 boniferous formation, Bancroft Place, NeAvport, R. I. (Walcott, U. S. Geol. Surv.) 



Reconstruction under Exceptional Conditions. 



Two special conditions greatly influence changes in rocks, viz., pres- 

 sure and heat. Their action gives rise to three general cases, but these 

 blend indefinitely: (1) exceptional pressure without great heat, (2) 

 great heat without exceptional pressure, and (3) great heat and great 

 pressure conjoined. Exceptional pressure may arise from the weight 

 of overlying rocks, or from lateral thrust due to the shrinkage of the 

 globe, and occasionally from other causes. Exceptional heat may 

 arise from pressure, from the intrusion of hot lavas, and occasionally 



