Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 69 



any tortuous course. The irregular and violent action, also, to which 

 the mass through which, according to this view of the subject, the 

 granite is supposed to have been protruded, would have a great ten- 

 dency, independently of the hydrostatic pressure just mentioned, to form 

 m the broken mass irregular fissures, which would facilitate the injec- 

 tion of the fluid matter, and increase the irregularity of the form of 

 the injected veins. 



§. On the Formation of Trap-Dijkes and Fehi.i. 



74. The results above obtained respecting the formation of fissures 

 m the crust of the globe will manifestly hold equally, whether we 

 suppose the uplifted mass acted upon immediately through the medium 

 of an elastic vapour, or by matter in a state of fusion in immediate 

 contact with its lower surface. In the latter case, however, this fused 

 matter will necessarily ascend into the fissures, and if maintained there 

 till It cools and solidifies, will present such phenomena as we now 

 recognize in dykes and veins of trap. The same phenomena would 

 result from the injection of the fluid matter at any period posterior 

 to that of the formation of the fissures as above described To repre 

 sent to ourselves, therefore, the phenomena of trap-veins, as referred 

 to the causes to which we are referring them, we have only to con 

 ceive the fissures previously described filled with trap The larger 

 ones will thus form dykes, and the smaller ones veins of that rock. 



75. It has been observed by geologists, and particularly by M'Culloch 

 that a large proportion of trap-dykes have been formed without pro- 

 ducmg any sensible disturbance in the ends of the stratified masses abut- 

 ting against them. And this is precisely what we might expect, if we 

 suppose such dykes to have been injected without excessive violence 

 mto fissures formed as above described, whether that injection be sup 

 posed to have taken place after the formation of the fissures, or con- 

 temporaneously with it. Where injection, however, has taken place in 

 great abundance, and with great violence, corresponding degrees of dis- 

 turbance might of course be expected to attend if. 



