76 Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



appears no reason to conclude that such has been the case. The whole 

 of Trotternish is described as consisting of an enormous overlying mass 

 of trap, which appears to have risen in numberless places through the 

 stratified rocks on which it reposes. It extends (if I understand the 

 description rightly) quite to the coast, so that scarcely any stratified 

 rocks are visible, except in the vertical section formed by the steep 

 cliffs along the beach, and in which the appearances above described 

 are observed. Hence it is 'probable that these horizontal beds are con- 

 nected with vertical masses of trap, at distances from the visible sections 

 of them, small in comparison with their apparent range along the cliffs, 

 and consequently it is very possible that the extent of horizontal injec- 

 tion may have been much less than at first sight it appears to have been. 



The same observation will apply to the other phenomena of the 

 same kind as described by the author just quoted; and so far from 

 offering any thing opposed to the theoretical views I have been ex- 

 plaining, they may, I think, be considered, when taken in conjunction 

 with numberless cases of vertical dykes and veins, as strongly corrobo- 

 rative of them ; since the comparatively insignificant number of these 

 injected horizontal beds, clearly proves them to offer only so many 

 exceptions to the very general rule of verticality in trap-veins, so fre- 

 quently recognized by M'CuUoch himself. 



81. In speaking of horizontal injection, I have not yet alluded to 

 the consequences of imperfect fluidity in the injected matter. If we 

 may be allowed to judge of the degree of this fluidity from the analogy 

 which the injected matter may be presumed to have borne to modern 

 lava in its eruption, we may conclude it to have fallen considerably 

 short of that of perfect fluidity. Consequently the lateral pressure 

 communicated by the fluid, would never be equal to the direct pressure 

 impressed upon it, and this, it is evident, would increase the difficulty 

 of horizontal injection in the cases which I have already considered. 

 The most important consideration, however is, I conceive, that this 

 property of imperfect fluidity, would thus impose a limit to the pos- 

 sible extent of lateral injection, supposing the injected matter not to 

 form a bed lying in one plane, but to form an irregular surface, such 



