164 W. H. HOBBS REPEATING PATTERNS IN STRUCTURE OF LAND 



The ultimate cause of this common type of deformation is presumably 

 the continued secular cooling of the planet. It is natural to look for an 

 explanation of the disorderly local fractures in local conditions — such, for 

 example, as the local buttressing effect of relatively rigid rock masses like 

 the Bohemian granite mass within the European architecture — or the 

 position of regions of elevation and denudation with reference to areas of 

 depression and loading. Such essentially local conditions may further 

 explain the considerable variations from complete parallelism between 

 lineaments of the same series either within any fracture field or between 

 distant fields. Noteworthy, however, is the rectangularity of the conju- 

 gate series within either of the two lineament sets which comprise the 

 pattern. 



There has been much discussion as to whether regional joints are ten- 

 sional or compressional in their nature. To the natural assumption that 

 they are generally compressional and a direct consequence of planetary 



Figure 37. — Diagram illustrating the progressive landtoard shifting of Strand Lines 

 with Uplift due to excessive Depression on the Ocean Floor 



The dotted line is ttie earlier and the full line the later profile of the surface 



contraction, there has been opposed the quite natural objection that being 

 connected directly with mountain-building, the earth's superfices must be 

 locally enlarged, and that tension and not compression is here demanded 

 by the conditions. This traditional difficulty has now been fairly met by 

 recent revelations from the "distant" study of earthquakes. It has been 

 shown that by far the larger proportion — probably at least nine-tenths — 

 of the heavy shocks, and by inference the larger molar displacements of 

 the crust, are suboceanic and connected with depression of the ocean floor. 

 The local dilation of the earth's surface, consequent on differential uplift 

 in mountains, must, therefore, be much more than compensated."^^ From 

 this it follows as a corollary that on a rising shore, strand lines should 

 not only be elevated, but should, further, move progressively landward 

 with the uplift (see figure 37). 



It has sometimes been a matter of surprise that regional joint systems 

 should have developed in late Pleistocene and in relatively "weak" ma- 



Proceedlngs of the American Philosophical Society, vol. 48, 1909, pp. 27-29. 



