458 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



the exception of exposures on the western edge of the mass, 

 there are hardly any at all to be found close to where the 

 junction is supposed to lie ; but these exposures on the west 

 are certainly very close, and must be allowed due weight. 



The most acceptable explanation of the facts would seem 

 to be a combination of bending and dislocation of the shales. 

 As the magma crept outwards and slightly upwards, a 

 point would be reached where the thickness of the shale 

 over the periphery of the intrusive sheet would have become 

 too thin for its cohesive strength to resist the upward lifts 

 of the magma. It would then bend at first, but as the 

 magma continued to rise it might very well be dislocated 

 at the furthest point reached by the magma, that is, where 

 the bent strata abut on the horizontal undisturbed shales # 

 This hypothesis is illustrated by the diagrammatic section 

 in fig. 3. In its favor the principal fact is the outward dip 

 of the upper junction near the periphery. The absence of 

 any existing shales with an outward dip is easily accounted 

 for by the extent of the denudation which has taken place, 

 which, as the line indicating the eroded surface will show, 

 must have entirely removed them. 



The mechanics of intrusions are as yet but very partially 

 understood. The magma may apparently either flow into 

 a cavity or fissure which is being opened by earth move- 

 ments, or it may make its own cavity, and this in several 

 ways. It may shatter the rocks, and dispose of the frag- 

 ments by assimilation or engulfing; 1 it may force the 

 strata gradually upwards, either by faulting, acting like a 

 hydraulic ram, or by folding, producing a kind of huge 

 blister as in laccolites ; or it may float up a block of strata 

 when it reaches a level determined by the relative specific 

 gravities of the country rock and the magma, as is supposed 



1 See Daly, Mechanics of Igneous Intrusion, Amer. Journ. Sci.,V"ol. xv, 

 (1903) p. 272. 



