FRACTURED APPEARANCE OF TRAP ROCKS. 71 



en masse after their eruption is concerned, but the appear- 

 ances visible in the position of the latter, prove that they 

 are not. 



Protruded, as the trap-rocks undubitably were, under great 

 superincumbent pressure, arising either from the presence 

 of an immense body of water or rock, it is evident that 

 they were not in a situation which could allow such a per- 

 fect motion of the particles on each other, as exists in a 

 stream of fluid rocky matter, erupted only under the pres- 

 sure of the atmosphere ; and such a great compressing force 

 might allow the ejected igneous matters to be piled up, one 

 above the other, in that more or less bedded arrangement, 

 which characterizes a trap-hill. The structure of the trap- 

 rocks also appears favourable for their having mural preci- 

 pices ; for in beds which have a tendency to a prismatic mode 

 of aggregation, and are composed of concretions arranged at 

 right angles to the plane of the bed, there appears to exist 

 a' disposition, which of all others is best fitted for producing, 

 in length of time, a cliff with a vertical front. Immediately 

 after the formation and consolidation of a trap-rock erupted 

 under the pressure of the ocean, it would begin to be sub- 

 jected to the destroying action of water ; and if raised 

 above the ocean, and forming dry land, would be subjected 

 to both the mechanical and chemical decomposing influences 

 of air and water. By whatever agents, however, the de- 

 composition of the mass was effected, it would proceed 

 chiefly in the direction of the vertical concretions, so that 

 whether the rock presented a mural front originally or not, 

 it would, when acted on by the various agents productive 

 of decay, assume one in a greater or less degree. By this 

 uniform mode of disintegration going on through a long 

 series of geological epochs, the original aspect of a trap 

 country might be modified to such a great extent, that there 

 might exist hardly any similarity between its former appear- 



