MOVEMENTS OF KOCKS, ETC. 789 



chasm must have had some considerable depth when these dykes 

 were formed*. 



The uniform width of dykes throughout such great heights as 

 are attained by those just mentioned t is manifestly a serious 

 difficulty in the way of theories of dyke-formation which suppose 

 the Assuring to have been caused by upheaval. It is almost equally 

 incompatible with any theory of fissuring by the pressure of the 

 intruded rock, supposed to act as a wedge ; for surely the displace- 

 ment of the material of a solid rock caused by the forcible intrusion 

 of a thick mass of trap into the lower part of it would produce 

 some torsional movement of the masses wedged apart, even sup- 

 posing it did not cause any upheaval ; and if there was a movement 

 of this kind, how was the parallelism of the sides of the fissure 

 preserved ? 



Again, the suggestion that the production of dykes proceeds from 

 the dragging apart of the solid crust by a stretching force is in 

 harmony with the fact that the deeper fractures from which igneous 

 flows take place have occurred where there was little folding, and 

 the more of the one, the less of the other. In the Appalachians, 

 where we find indications of great lateral compression, no such 

 outflows are knownij:. 



While, however, the bending of strata is not the immediate cause 

 of the Assuring which has produced dykes, it is evident that in many 

 cases it may be the cause of fractures or joints, which afterwards are 

 converted into fissures by the spreading of underlying molten matter 

 in the way suggested §. 



That the body of trap injected is, in some cases, relatively so large ||, 

 is no difficulty in the suggested explanation. Where this is so we 

 shall argue the presence of a large mass of molten matter beneath 

 the crust at the time of the injection, and a large spreading 

 movement of this mass. 



It is, however, otherwise with theories of fissuring by contraction. 

 Por where numerous cracks or fissures are produced in a substance 

 by unequal contraction due to unequal cooling, they always have a 

 relatively small magnitude ; and to account for large dykes in this 

 way it is necessary to make some additional supposition, such as, 

 that the molten matter exerts hydrostatic pressure laterally — a 

 supposition very difficult to allow, when we find no effect of such a 

 pressure in an upward direction, that is, in what is generally the 

 direction of least resistance. 



Most, if not all of the effects which I have thus far endeavoured 

 to connect with horizontal movement produced by gravitation are 

 displayed in a particularly instructive manner in the singular group 

 of mountains in the Plateau Province of the American Union known 

 as the Henry Mountains. 



* Tertiary Hist, of the Grand Canon District, p. 95. 

 t See also Scrope's ' Volcanos,' p. 165. 



I Dana's ' Geology,' p. 791. Dana says that a lateral pull rather than a lateral 

 pressure is apparently required for the origin of some dykes. Ibid. p. 803. 



§ Dana's ' Geology,' p. 803. 



II See Macculloch, 'System of Geology,' i. p. 110. 



