80 Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



referrible to some cause totally distinct from the action of extraneous 

 forces on the general mass. This, however, forms no argument against 

 our theory, as applied to those cases in which the phenomena present 

 to us features entirely different from those just mentioned, and in per- 

 fect accordance with our theoretical deductions. 



With the causes which may have superinduced the jointed structure 

 in rocks, I liave at present no concern, except so far as it might possibly 

 be influenced by the action of extraneous forces. It has been shewn, 

 however, (Art. 32), that such forces could only tend to produce systems 

 of fissures crossing each other at right angles, whereas regular systems 

 of joints appear to meet each other frequently at acute angles, and 

 consequently, must necessarily have been owing to some different cause. 

 I do not therefore conceive that any general tension of the mass pro- 

 duced by extension from elevation, or contraction in the course of 

 solidification, can have had any material effect on the formation of joints. 

 It is probably, I think, to be referred entirely to some kind of internal 

 molecular action. 



Though the law of approximate parallelism has long been recognized 

 by geologists as characterizuig mineral veins, faults, &c., I am not 

 aware that any attempt has hitherto been made to deduce this important 

 law from the causes to which these phenomena have been referred. In 

 the preceding investigations, however, I have shewn, that under certain 

 simple conditions, such a law is the necessary consequence of a general 

 elevatory force acting in the manner I have supposed; and I have 

 moreover shewn, that this law is entirely inconsistent with the partial 

 action of such a force; because an elevatory force acting thus partially 

 at a particular point, would necessarily produce fissures diverging from 

 that point, so that in a general elevated range produced by the eleva- 

 tion of different portions in succession, there could be no general system 

 of parallel fissures. This deduction appears to me perfectly conclusive 



