It. A. Daly — Abyssal Igneous Injection. 203 



So long as folding or overthrusting of the shell of compression 

 does not occur, the two shells are in physical continuity and 

 are strongly bound together. 



Secular accumulation of tensions and of cooling cracks. — 

 It is generally agreed that, on the contraction theory of moun- 

 tain-building, orogenic folding and crumpling is possible through 

 the secular accumulation of compressive stresses in the outer 

 shell. The crucial question has not yet been satisfactorily 

 answered as to whether there may be similarly a secular accu- 

 mulation of tension and of its effects in the inner shell of the 

 crust. If the crust were a fluid of high though finite viscosity, 

 the accumulation of tension would be impossible to any sensible 

 extent ; moreover, the weight of the crust overlying any sub- 

 shell would necessarily close all cavities almost as fast as formed 

 during the slow secular cooling. But the average rock of the 

 crust is a true solid known to have a very low modulus of j^las- 

 ticity. Pfaff has, indeed, denied even the smallest measure of 

 true plasticity to the average crust-rock, and his experiments 

 seem to prove that massive rocks like granite, gneiss or gabbro 

 would, at surface temperatures, not flow under the weight of 

 even 25 miles of overlying rock.* They would rupture and 

 shear, but the deformation would not reach the perfection of 

 the molecular shearing implied in true flow. 



A vertical crack due to cooling contraction would thus tend 

 to be partly closed by shearing in of masses from its walls. 

 The shear-planes would be inclined to the vertical. Each 

 partial bridging of the crack makes further shearing and 

 closing of the crack more and more difficult. A greater weight 

 of crust would now be required since some support of the load 

 is formed through the local meeting of the solid walls. ' The 

 simple vertical stress becomes partially resolved into a com- 

 plex network of oblique stresses tending to balance each other 

 in the loci of lateral support. The portions of the crack 

 occurring between these loci of support may remain open 

 because of the diminished shearing stresses along the still 

 gaping walls. It thus appears that, though all rocks at surface 

 temperatures will rupture under the weight of less than 6 

 miles of crust, yet the complete closing of cracks at the same 

 temperatures would not be expected even under the weight of 

 a much greater thickness of crust. The deptli of the shell 

 ("zone") of fracture has been deduced from the crushing 

 tests of stone and from the brilliant experiments of Adams 

 and Nicolson on the deformation of marble enclosed in steel 

 collars. The former tests evidently do not prove anything at 

 all definite as to the pressures required to produce true plastic 



*See Adams and Nicolson, Phil. Trans. Eoyal Soc. London, vol. cxcv, 

 p. 367, 1901. 



