70 Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



The geologist to whom I have just referred, in speaking of the 

 trap-veins of the Isle of Sky, observes: "It is necessaiy to point out 

 one extraordinary effect which must have resulted from the intrusion 

 of these veins. Whatever proportion, collectively taken, they may bear 

 in breadth to the lateral dimension of the strata which they intersect, 

 it is plain that the whole mass of strata must have undergone a lateral 

 extension equal to that quantity ; a motion so great as not to be easily 

 reconciled with the present regularity of the whole. It is also a singular 

 circumstance, that on the opposed shore of Sleat a different effect takes 

 place, and proportioned, it would here seem, to the number of veins; 

 the red-sandstone strata of this coast being often turned from a slightly 

 inclined into a nearly vertical direction, with other considerable marks 

 of disturbance. It is impossible to account for these apparently capri- 

 cious differences, and we must for the present be content to rank them 

 among the numerous unexplained phenomena in which the science 

 abounds." 



These phenomena present no difficulty except in the apparent lateral 

 displacement of the stratified beds, without any other appearance of 

 disturbance; and if this effect is to be referred to the lateral pressure 

 of the injected matter, it does indeed present a difficulty no less, I 

 conceive, than a physical impossibility. In the first place, it appears 

 inconceivable how sufficient resistance could be obtained from above to 

 produce the enormous lateral fluid pressure necessary to cause this 

 lateral movement, as we have before remarked respecting the horizontal 

 heaves of mineral veins; and in the next place, it is still more incon- 

 ceivable how this force could have been exerted without indications of 

 such violent action. Under the point of view, however, in which I have 

 regarded the subject the difficulty no longer exists ; for it must be 

 recollected that the aggregate width of the veins, or apparent lateral 

 displacement, is not to be taken with reference to the breadth of the 

 mass in which the veins immediately exist, but with reference to the 

 whole extent of the mass, the tension of which may have been relieved 

 by the formation of these fissures. No rational account can be given, 

 I conceive, of such lateral movements of extensive masses, except by re- 

 ferring them to the horizontal tension produced by vertical forces, and 



