EXPLAXATIOX OF PLATES 301 



thetical controlled expansion" represents the effect of heat in terms of cubical 

 expansion under increasing pressure. The "Hypothetical resultant" is the 

 difference between the two, and is subject to instantaneous change if either 

 pressure or temperature vary. 



Plate 10. — Illustration of Deformation of a Solid in the Zone of Floicage, 

 solid Floir heing accompanied by Shearing 



Plate 10 represents shearing in substances of the consistency of butter at 

 20° C. The effect was produced by compressing a model composed of layers 

 of wax. mixed with thick Venice turpentine to soften it or with plaster of 

 Paris to harden it. 



The lower layers, in which shearing is conspicuous, were soft and flowed 

 under the load of shot which rested on them and under the pressure which 

 was applied at the right. AYhen a plate-glass covering the face of the model 

 cracked this soft material flowed out through the cracks to a height of half 

 an inch. It nevertheless sheared on planes at 45° to the stress. During the 

 movement the earlier shear planes were revolved. The later ones at the left 

 retain their initial orientation. The harder upper layers, above number 8, 

 yielded by flexure rather than by shear. (See Experiment J. Plates XCIII 

 and XCIV, Thirteenth Annual Report. U. S. Geological Survey, 1889.) 



Plate 11. — Positive and negative Elements of North America 



Plate 11 represents the continental platform of North America and the 

 oceanic deeps adjoining it, the whole being shaded so that the darker areas 

 correspond with depressions and the lighter ones with elevated tracts. Be- 

 neath the oceans the shading represents the actual relief. On the continental 

 platforms the shading represents a hypothetical relief which would exist if 

 there had been no erosion, neither degradation nor aggradation, since the 

 beginning of the Paleozoic. The relief which would exist if that assumption 

 were true is deduced from the sedimentary record, according to the algebraic 

 sum of .sediments and unconformities, expressed in epochs, for each separate 

 area. Where erosion has been constantly or commonly active the correspond- 

 ing area would now stand high. Where sediments have commonly been de- 

 posited the surface under them had sunk low. The continent is thus divided 

 into masses that have tended to stand high or to sink relatively low. 



The tendency which the several areas have exhibited throughout known 

 geologic history is, under the hypothesis of isostasy, attributed to their rela- 

 tive lightness or to their greater density. The lighter masses are shown as 

 white areas. They are the positive elements of the continent. The heavier 

 masses are shown in medium and dark shades. They are the negative ele- 

 ments of the continent. The former are the interdisks, the latter the disks, 

 according to the definitions of those terms under the discoidal hypothesis. 



The names given to the positive elements are tho.se employed by Willis 

 (1907) or by Schuchert (1908) to designate masses in a similar locality. 

 Likeness of outline is not to be expected or striven for in the present state of 

 paleogeography. Mexia. Klamath Land, Wrangell Land, and some others are 

 new names. 



