F. H. Lahee — Metamorphism and Geological Structure. 253 



rule that pitch is practically universal in regions of folding* — 

 the deforming forces were variable in direction and they 

 may be considered as resolvable into two components as above 

 explained. The ratio between these components will then be 

 indicated by the ratio between the degrees of compression 

 perpendicular to, and parallel to, the axis of the given fold. 



According to Yan Hise,f folding may be parallel or simi- 

 lar. In parallel folding the contacts between adjacent beds 



Fig. 3. 



Fig. 4. 



Fig. 3. 

 Fig. 4. 



Parallel folding. 

 Similar folding. 



A-B, fundamental curve. 



are parallel to one another and the thickness of any bed is 

 essentially uniform throughout (see fig. 3). On the other 

 hand, in similar folding the contacts between adjacent beds are 

 identical in size and shape and the thickness of every stratum 

 is considerably greater in the axial regions than on the limbs % 

 (see ^. 4). Again, in parallel folding that curve in which 

 all anticlines and synclines are of equal size and shape may be 

 termed fundamental to the structure (A-B, fig. 3). Above 

 the fundamental curve synclines narrow and become 'pinched' 

 or 'carinate', and below it anticlines undergo the same altera- 

 tion in form. Obviously no such discrimination is possible in 

 similar folding. It may be shown that, while deformation is 

 in process, differential movement (shearing) near the funda- 

 mental curve is at a maximum in the limbs and at a minimum 

 in the crests and troughs ; but that, away from this curve, 

 both upward and downward, the locus of maximum differen- 

 tial movement migrates from the limbs to the axial regions, 



* See, for example, the following: Reade, T. M., The Origin of Mountain 

 Ranges, xxx, p. 178. London, 1886. Reade, T. M., The Evolution of Earth 

 Structure with a Theory of Geomorphic Changes, p. 195. London, 1903. 

 Shaler, N. S., etc., op. cit., p. 32. Van Hise, C. R., Deformation of Rocks, 

 Jour. Geol., iv, pp. 312, 344, 348-349. 1896. 



f Van Hise, C. R., Principles of North American pre-Cambrian Geology. 

 U. S. G. S., Ann. Rept. XVI, Pt. I, 1894-1895, pp. 598, 599. Also, by the 

 same author, Deformation of Rocks, Jour. Geol., iv, pp. 210, 211, 1896. 



J Van Hise, C. R., Principles of North American pre-Cambrian Geology, 

 pp. 598-601. Also, Heim, A., Untersuchungen iiber den Mechanismus der 

 Gebirgsbildung, Basel, 1878, p. 48. 



