454 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 
breaking it in its passage as a solid wedge might, but an actual injection or pump- 
ing of it into the newly opened vacuous cavity, from the pressure or tension below. 
Fig. 6. 
Dykes expanding upwards in Anticlinals and downwards in Synclinals. 
Intrusion of the Igneous Rocks in Solid Wedges. 
The other notion, frequently connected with the above idea, of a forcible pro- 
pulsion of igneous matter through the crust, is that of the violent thrusting 
upward of volcanic or granitic matter already solidified, in broad wedge-like masses 
through the strata. This conception I hold to be at variance both with sound 
mechanical laws, and with the physical facts. For the solid igneous mass to have 
acted in the manner of a wedge, it is absolutely necessary that it should have 
moved freely upward through the opening in the strata, which it is supposed to have 
wedged apart and to have uplifted, and even corrugated, by lateral compression. 
But it is impossible to imagine such a slipping of the assumed granitic wedge 
past the edges of the strata confining it, since we can imagine no force acting 
downward upon these latter, to prevent their moving upward along with the 
wedge of granite, nor any localization of the force below, to prevent it operating 
on both alike. We have furthermore no evidence of that discontinuity between 
the igneous rock and the ruptured strata, which the notion of a sliding wedge 
obviously presupposes; but, on the contrary, every proof from general theory and 
from observed facts, that the two descriptions of rock are intimately bound to- 
gether in closest crystalline contact, keyed together by veins, branching from the 
mass of the one into the fissures of the other, and even fused together by an 
actual incorporation of substance. Any upward movement, therefore, of Plutonic 
masses, bearing sedimentary rocks upon their flanks, cannot have been in the 
manner of a mechanical wedge; and those results—corrugation for example—of 
the adjacent strata habitually attributed by many geologists to an imagined 
wedge-like lateral thrust, must be accounted for upon some other sounder me- 
chanical theory. 
A modified form of this conception of an igneous wedge lifting and displacing 
the strata, assumes no sliding or wedge-like protrusion of the solid granitic matter 
past the edges of the rupture in the bedded rocks, but recognizing the inseparable 
cohesion of the two, regards the stratified masses flanking the anticlinal mountain, 
as merely borne upward by the uprising of the central igneous nucleus. I deem 
this notion to be a much truer picture of the procedure of nature; for it so far 
accords with what we notice in anticlinal districts having igneous crests or centres, 
