HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 131 



the shearing-zone may well be supposed to have given rise to that alternation of 

 sheets of igneous rock and foliated rock which constitutes a common and typical 

 phenomenon of the Archean terranes. It may well be supposed further, that the 

 more viscous igneous rocks, by gradually stiffening, might often lodge in the more 

 nearly horizontal stretches of the shearing zone, in large, flattened, disk-like lenses, 

 and constitute batholiths. The enormous areas occupied by the Archean batho- 

 liths seem to find in this an explanation that does not involve the serious dis- 

 placements of adjacent rock usually assigned to them. 



Secondary or tensional movements. — In the foregoing primary deformations 

 the movements are essentially compressive in effect. Incidentally, some tension 

 is involved in particular portions, as on the tops of folds, and perhaps en the great 

 arches of the plateau- swells, and elsewhere. These primary processes therefore offer 

 no adequate explanation of the prevalence of tension in the outer part of the 

 continents. As these tensional phenomena are widely prevalent, they imply a 

 general cause. They are here referred to secondary action, following upon the 

 primary deformation. In the sinking of the great sectors and in their mutual 

 crowding, especially in the formation of plateaus, it is held that portions of the 

 continent were relatively elevated above the plane of isostati" equilibrium, and, 

 as the mass of the interior, though solid, is not absolutely rigid, there is ground 

 for the presumption that the excess of gravity in the protuberant portions was 

 gradually relieved by their slow sinking; in other wcrds, a secular movement 

 in the direction of isostasy would follow the periods of deformation. This view 

 is especially applicable to the great plateaus. A direct movement downward, 

 which would be the primary phase of an isostatic movement, would, however, 

 tend to develop thrust, but because of the protuberance a lateral creep is pos- 

 tulated. Such a creep is postulated regardless of isostasy. The continents now 

 rise about 3 miles above the average ocean bottom, and much more than that in 

 certain portions. Since the volume of the ocean has probably not greatly changed 

 during known geological time, and since the continents appear to have risen to 

 and above the surface, about as now, in the earliest known geological ages, it is 

 assumed that similar differences of relief have prevailed at the various known 

 deformative periods. We have seen that the gravitative pressure at 3 miles 

 depth is nearly or quite as great as the shearing-resistance at that depth. If a 

 shearing-zone at depths of 3 to 5 miles had already been developed during the 

 primary deformation, this would facilitate a reversed movement along the same 

 shear -planes. It is therefore conceived that the continental platforms, by press- 

 ing upon their bases to the extent of 16,000 to 30,000 pounds to the square inch, 

 would, though opposed by perhaps 5000 pounds per square inch pressure from 

 the oceans, gradually creep laterally under their own gravity, in a slow glacier- 

 like way. This supposed lateral creep should be attended by crevassing, Assuring, 

 and normal faulting; in other words, by the 'prevalent tensional phenomena which 

 the continents present. It should be much more marked in the plateau regions 

 than elsewhere, because of their superior elevation, and it is there that normal 

 faulting is most pronounced. 



Horsts, rift-valleys, etc. — In a tensional movement of this kind, affected by 



