256 DISLOCATIONS OF STRATA 



particles are again welded together into a firm mass. We may 

 accordingly distinguish a zone of flow age, in which the rocks all 

 yield plastically, a more superficial zone of fracture, in which all 

 but the softest rocks break on compression, and between the two 

 a zone of fracture and flowage, in which some rocks break and 

 others flow, according to their rigidity. The depth of the zone of 

 flowage is estimated at 20,000 to 30,000 feet below the surface. 



Strata which have not been buried to a sufficient depth to 

 make them plastic, will yield to compression by breaking, though 

 whether a given bed is faulted or flexed, will often depend upon 

 whether the folding force is applied slowly or with comparative 

 rapidity. A force long acting in a slow and steady fashion will 

 produce folds, when the same force applied more suddenly would 

 shatter the beds. Near the surface, under light loads, rigid rocks 

 will always break rather than bend, when compressed. Different 

 stratified rocks differ much in their rigidity, and hence a load 

 which is sufficient to cause one bed to bend and flow, when later- 

 ally compressed, will leave another unaffected, or cause it to 

 break, if the compressing force overcomes its strength. In Bald 

 Mountain, New York, the stiff limestones are left unchanged by a 

 pressure which has crumpled and contorted the soft shales. 



Another factor of much importance in determining the char- 

 acter and position of folds is the mode in which the strata were 

 originally laid down. As we have already learned, the sheets of 

 sediment which cover the sea-bottom are, on a large scale, nearly 

 level, but they often show slight departures from such horizontal- 

 ity along certain lines. These initial dips often determine the 

 place of flexures, because they divert the thrust from its horizontal 

 direction. 



The effect exercised by initial dips is shown in Fig. 112, taken 

 from the models experimented on by Mr. Willis, which, when 

 strongly compressed, imitate with remarkable accuracy the struct- 

 ures which may be observed in folded rocks. Fig. 11 1 shows that 

 in folding, the beds must slip upon each other, as is proved' by 

 the lines perpendicular to the bedding planes, which were contin- 

 uous before folding, but in the anticline are broken by the differ- 



