298 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERE 



and an actual bowing down of the roof must follow, resulting in tension 

 in the lower layers of the gneiss. The development of shearing planes 

 and the intrusion of dikes along them would ensue. Thus a bonded 

 structure of gneiss intruded by basic magma would be built up. 



Assuming that the intrusive cools, it is evident from the discussion of 

 melting conditions that on reheating it must melt first. We may sa}^, 

 once an intrusive, always an intrusive. Thus the sheets and dikes of 

 basic rock will extend themselves by repeated melting and intrusion. On 

 reaching the surface the dike becomes the feeder of a flow, and in one or 

 several epochs of eruption builds up a mass of heavy basic rock. 



The essential point upon which emphasis should be laid is that the 

 higher conductivity and lower melting point of basalt make inevitable 

 the remelting of basaltic masses and the repeated extrusion of tiie magmas 

 along the same conduits. Two conditions may arise to stop or divert the 

 outflow. The first is the exhaustion of the magma basin. When all the 

 basic magma had been pressed out the gneiss would close on itself and 

 basic eruptions would cease. The second condition is horizontal, inclined, 

 or vertical faulting, which may displace a dike and occasion the opening 

 of new channels to possibly remote exits. 



Now the extrusion of heavy masses to or near to the surface produces 

 a disturbance of isostatic equilibrium. As Gilbert has shown, the sur- 

 face gravity is thereby intensified. The underlying gneiss would be sub- 

 jected to excessive vertical load, which would produce lateral, outward 

 horizontal stress and readjustment by recrystallization, either during the 

 epoch of intrusion or during a later heating. The readjustment requires 

 a lowering of the surface and the development of a corresponding depres- 

 sion. 



If now we review the postulates and steps of the above outlined process, 

 it is clear that the magnitude of the depression depends upon the size of 

 the original basalt mass and the repetition of epochs of melting. It is 

 not unreasonable to assign to the original mass of basic magma diameters 

 approaching those of the oceanic deeps, which are parts of the ocean 

 basin, and thus to suggest the origin of the basins themselves. 



The basic magma rises, according to hypothesis, from the astheno- 

 sphere, that is from depths of 60 to 800 miles below the surface. A 

 direct path along vertical foliation planes might be established in the 

 •early stages of the rise of the magma, but it would be replaced by inclined 

 shearing planes and horizontal foliation during subsequent stages. An 

 important group of shearing surfaces would be peripheral and inclined 

 outward from below upward. The magmas following up those surfaces 

 would form the outer limits of the complex bonded structure, which 



