lyo WILLIAM H. HO BBS 



developed in series, and a fruitful inquiry would be to fix, by 

 thorough examination of folded districts, the maximum span of 

 fully disclosed, as against merely inferred, anticlinal arches. 



Importance of lenses of sediments in inducing folding. — Lest too 

 large importance be ascribed to local weakness of strata in fixing 

 the location of initial folds, it is well to remember that folded areas 

 do not appear to be those which are thinnest, but, on the contrary, 

 that they are the thick lenses of sediments in which formations are 

 present in fullest development — areas of continuous deposition in 

 former epi-continental seas. The explanation of this fact is found 

 in the peculiar cross-section of a lens of sediments. Obviously a 

 perfectly straight rod of more or less rigid material whose axis is 

 parallel to the direction of compression will transmit larger stresses 

 than one which may be considerably thicker and stronger but is 



initially sHghtly bent from 

 ^ . ), the direction of the compres- 

 sive stress (Fig. 12, a and h). 

 7 In the first instance (a), the 

 applied force being directly 

 opposed by the resistance. 



Fig. 12. — Diagram to illustrate the rela- the rod tends merely tO be 

 tive tendency to fold, or buckle, of: a, a sHghtly thickened, whereas in 

 straight rod; and b, a thicker but slightly /7\ i 



curving rod. ^^^ second case (0) , the active 



force is deflected parallel to 

 the tangent to the curve, and at a rapidly accelerated rate this 

 deflection is augmented with increasing pressure. It is thus easy 

 to see that the cross-section of basins of deposition, being lenticular, 

 may have been more important in favoring the location of folds than 

 their greater thickness could have been as a hindrance. WilHs, who 

 discovered the importance in the location of folds of what he has 

 called "initial dip," has thus stated the law of competent structure: 



The transmission of pressure through a folding, stratified mass may be 

 stated as follows: So long as the stratification is parallel to the original direction 

 of pressure, the force is transmitted as a whole and tends to reduce the volume 

 of the mass; when the strata are inclined to the direction of pressure the thrust 

 is resolved into two components, the one parallel to the bedding, the other 

 perpendicular to it; the former produces movement when it overcomes the 



