130 GEOLOGY. 



tioned, as will appear later, they are conceived to belong to a dynamical category 

 different from that of the great plateaus here under discussion. In the formation 

 of the latter, while the whole outer portion of the earth is supposed to have been 

 descending, the plateaus included, the parts mutually crowded upon one another, 

 resulting in the relative displacement of some of them by processes of wedging, 

 squeezing, and mass distortion. The dynamics here were positive, so far as the 

 parts were concerned, not negative, as in the case of true horsts. 



The superficial shell, not having undergone the molecular and thermal changes 

 assigned to the interior, and hence not having shrunk proportionally to it. is 

 supposed to have accommodated itself to the shrunken interior by thrust, as 

 previously described, and as commonly held under all hypotheses. Fig. 32c 

 illustrates graphically the supposed movements acting conjointly. 



The zone of accommodation. — Between the folded shell and the interior, 

 where flowage-deformation prevails, there is presumed to be a zone of accom- 

 modation by which the one is adjusted to the other. This zone of accommodation 

 is supposed to be affected partly by shear and partly by flowage, depending on 

 the nature of the formations of which it is composed at any point, and on the 

 nature of the stresses which there affect it. 1 It is presumed to graduate from 

 shear along a few selected planes above, through shear along many planes in the 

 middle, to continuous shear, or flowage, below, and thus to constitute a transition- 

 zone from the shell to the main body. 



The shear-zone a zone of foliation. — The more distributive phases of the shear 

 in this zone would obviously give rise to foliation and schistosity. Structurally, 

 therefore, the shear-zone becomes a zone of foliation, practically enveloping the 

 globe next beneath the rigid shell. Under the planetesimal hypothesis, this zone 

 is supposed to have been gradually lifted as the growth of the earth progressed, 

 and hence to have undefined depth. In those portions of the continent that 

 were protruded above the sea at an early date, and that were degraded to give 

 material for the sedimentary rocks, it is conceived that this zone has been ex- 

 tensively exposed, and constitutes, in part at least, the foliated rocks now known 

 as Archean; indeed the Archean areas are conceived to be essentially those 

 portions of the shearing zone that have been brought to the surface by the removal 

 of the shell that once overlay them. 



The relations of the shear-zone to igneous intrusions. — From Fig. 32c it will 

 be seen that the shearing-planes are flexed as a result of lateral movement, and 

 that, typically, their section is a double curve, the middle portion of which may 

 not be greatly removed from the horizontal. It is conceived that molten rock, on 

 reaching this zone in its passage outward from the interior, would be predisposed 

 to follow the shearing-planes already developed, these being the lines of least 

 resistance. The rising molten rock must often have been solidified by cooling in 

 the course of its passage, and hence residual portions arrested between the sheets of 



1 Technically, flowage takes place by shear, but it is the shear of each part on even- 

 other adjoining part, while in the more common usage here followed, the shear is 

 confined to selected planes. 



