OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 453 



violently thrust in solid wedge-shaped masses upwards through the incumbent 

 crust. That this is the prevailing idea, is apparent from the manner in which 

 nearly all geological sections, even the most modern ones, designed to represent the 

 relations of the Plutonic to the sedimentary rocks, are to this day constructed. 

 Where igneous rocks constitute the whole, or a large portion of the central axis of 

 a mountain chain, or even that of a simple anticlinal ridge, they are usually 

 represented in cross sections, in the form of a broad wedge, and the stratified rocks 

 are drawn as leaning upon the sloping flanks of the wedge or prism. This is not, 

 I think, the true relation in Nature of the igneous to the sedimentary masses, as I 

 propose to show from the following considerations. 



Hypothesis of Wedge-like Intrusion of Melted Matter. 



The notion of an upward wedging, or intrusion of molten mineral matter into 

 or through the superincumbent strata, in the manner of a wedge, implies a func- 

 tion in the soft material which belongs to the mechanical action of a solid, and 

 is incompatible with the dynamic properties of fluids. Until a fissure from below 

 first penetrates or traverses the invaded overlying strata, it is not possible to con- 

 ceive, that the liquid matter could introduce itself in the mode of a wedge. Some 

 force must first crack the crust, and then the molten matter, flowing into the fissure 

 may act as a narrow wedge or key, to keep the walls of the chink distended, but 

 such plates of solidified refrigerated volcanic matter, known as veins and dykes, 

 must necessarily be narrow, and have the shape rather of walls with parallel sur- 

 faces, than great wedges broad at the base.* They will also abound chiefly in the 

 districts of subsidence, or in the concave waves, not in those of elevation, or in 

 the convex, where the wedge-like form tapering upwards, is usually represented. 

 Where a rupturing of the strata has taken place in a tract of elevation, or at 

 an anticlinal, the fissure or fissures will be found to gape upwards, and the 

 melted volcanic matter which has flowed to the surface, will be seen widen- 

 ing outwards and tapering as it descends, the very opposite of the form usually 

 assigned to such outbursts, in the igneous axes of uplifted chains. So common 

 is this upward enlargement of the Plutonic masses in certain regions, that it con- 

 stitutes, I conceive, one element of the fan-shape or inward dip of the boundary 

 walls of the rocks, so frequently encountered in the Alps and other much dis- 

 turbed mountain systems. A true conception of the formation of a mineral vein 

 or dyke will represent it as the consequence, not the cause, of the fissure which it 

 fills, the real process being, not a protrusion of the fluid matter through the crust, 



* It is in consequence of this natural expansion of surface- cracks outwards in anticlinals, that 

 the miner so frequently finds his mineral lodes contracting or dying out as he descends. Several 

 striking instances of this thinning of veins downwards could he cited, from the mines of the United 

 States, situated in anticlinal flexures. 



