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, 



