MOVEMENTS OP EOCKS, ETC. 787 



Any such yielding of the solid rocks around to the pressure of the 

 molten rock will tend to draw apart the solid crust resting upon it ; 

 and thus if the crust is not too strong, we shall have the ground 

 openiug along lines of weakness such as are produced by the presence 

 of joints or other close fractures, and more or less extensive fissures 

 will be formed. And in some cases, where there is any considerable 

 adhesion of the crust to the spreading mass beneath, and the spreading 

 is great in amount, the crust may be expected to break up into larger 

 or smaller fragments, much as the ground-surface breaks up and 

 separates in the case of landslips. 



In cases where the quantity of molten matter spreading is large, 

 relatively to the thickness of the solid crust, and the conditions are 

 such that the mass spreads considerably in seeking equilibrium, the 

 force operating to extend and rupture the crust will, it is evident, 

 be both great and of long continuance. In all cases the degree of 

 viscosity of the molten matter, and the degree of plasticity of the 

 solid crust, and the presence of joints and fissures will all bo impor- 

 tant factors in determining what effects are produced *. 



Next with regard to the production of trap-dykes. When a large 

 mass of molten matter is present near to the surface, and a fissure is 

 produced in the manner referred to, the weight of the ruptured crust 

 will, if the plastic mass beneath be liquid enough, cause the latter 

 to rise in the fissure, either as it forms or immediately after its 

 formation. 



The view that the production of the fissure precedes and is distinct 

 from the extravasation of the matter forming the dyke, and that the 

 latter is due to a relatively gentle hydrostatic force not capable of 

 driving the lava into and through solid rocks, is supported by the 

 fact that in many volcanic eruptions, lava flows out quietly and with- 

 out explosive violence f ; also by the fact that a subsidence of the 

 strata around volcanic vents, such as would follow the hydrostatic 

 movement of the lava, is sometimes seen J. 



Dutton tells us that '•' a careful examination of the details of vol- 

 canic eruptions leaves the impression that they are pressed up by 

 the weight of rocks which overlie their reservoirs, and that their ex- 

 travasation is merely a hydrostatic problem of the simplest order "§. 



The rending of the rocks preparatory to the extravasation of molten 

 matter has, according to the view I have submitted, commonly taken 

 place with suddenness and on a large scale. And I think we have 

 evidence that this has been so in the case of many dykes, in the 

 familiar fact that they generally take their course without regard 

 to the irregularities in structure and disposition of the masses they 



* An instance of a large body of solid rock, which overlay molten rock, 

 shifting in the manner suggested, is given by Mr. Dana : see his ' Geology,' 

 3rd edition, p. 731. 



t Scrope's ' Voleanos,' 1872, p. 160. 



X Scrope's ' Volcanos,' 1872, p. 228. The formation of gases and heat-expan- 

 sion of rock which occur beneath volcanic vents will, it is evident, operate to 

 produce elevation of the crust ; and it is not therefore surprising that subsidence 

 should be observed but seldom. Ibid. p. 226. 



§ ' Geology of the High Plateaus of Utah,' p. 130. 



