1855.] FORBES FOLIATION OF ROCKS. 179 



sideration, as being in some cases the agent in causing as well as 

 modifying foliated structure. 



Whatever may he the cause of the arrangement of foliation in cer- 

 tain directions, I believe that most geologists agree in supposing that 

 fohation itself is due to the action of heat, which has reduced the 

 rock in situ to a fluid, or at least semifluid state, to which some add 

 the action of gases and pressure. 



On this point I would wish to make some remarks ; and I may 

 premise by stating, that to me foliation appears to be the result of 

 chemical action in recombining the elements existing in amorphous 

 sedimentary rocks, together with a simultaneous molecular move- 

 ment of the products thus formed ; also, that this action is efi'ected 

 by heat, but has taken place at temperatures lower than even neces- 

 sary to change the external form of the masses, or to produce any 

 semifusion or even softening. 



As corroborative of this, I may refer to fig. 10, which Keilhau 



Fig. 10. — Section of the gneiss in the Island of Jomfruland^ 



Norway. 



has also observed and made mention of*. It represents an appear- 

 ance in the gneiss on the island of Jomfruland. 



Here we have a vein of hornblendic character running across the 

 gneiss, and disturbed by the fault AB, which throws it downwards. 



On examining the lines of foliation, it will now be found that those 

 lines which are most distinct are not at all affected by the fault, and 

 continue throughout with the greatest regularity ; so that it can 

 hardly be doubted, that the foliation of the gneiss took place later 

 than the formation and subsequent dislocation of the hornblende vein ; 

 and consequently we cannot suppose that the mass could have been 

 in a fused or softened state at the time of fohation without the 

 obliteration of these appearances. Again referrmg back to fig. 8, 

 p. 177, it will be seen that the fault b, the origin of which apparently 

 was previous to or simultaneous with the granite-vein, has altered the 

 position of the foliation on the one side and bent it upwards. The 

 faults c, D, and e do not effect this at all. 



If, however, the rock had been in a semifused state when foliation 

 took place, or when the granite-vein was injected, we should have 



* Norske Magazin for Naturvidenskab, vol. iii. p. 175. 



o2 



