Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



73 



would be exceedingly increased by the consequent additional pressure, 

 while the process of injection would not be in the smallest degree facili- 

 tated by it. Trap-veins would thus be produced, affording indubitable 

 evidence of injection. 



Again, the hypothesis we have made above of the entire absence of 

 adhesion between two contiguous beds, though it may in some cases 

 be true for limited spaces, cannot be uniformly so in cases in nature 

 for spaces of considerable extent. Now in those instances, in which 

 the force of adhesion between the two beds, bears any kind of ratio 

 to that which holds together the component particles of the uplifted 

 portion, an enormous force will be required to overcome this adhesion. 

 And how are we to conceive such a force applied without producing 

 the smallest rupture in the lower surface of the uplifted mass? If 

 there be no adhesion between the beds, no considerable horizontal 

 tension will be produced in this mass ; but if the adhesion be con- 

 siderable, such a tension will be produced, proportional to the increased 

 force of injection called into action. Under these circumstances the 

 smallest break or crevice will be torn open, the fluid matter will 

 enter it, and acting on its vertical sides with an enormous pressure, 

 and with the mechanical advantage of a wedge, will add immensely to 

 the tendency of the horizontal tension to produce a vertical fissure. 



78. It may perhaps be thought that the difficulty of conceiving 

 the process of horizontal injection of considerable extent, without the 

 production of vertical veins, may be obviated by supposing the fluid 



matter injected from many points simultaneously between the same two 

 horizontal beds. But this hypothesis appears extremely improbable. 

 Vol. VI. Part I. K 



