307 



the next above the first was the second that was formed, and so on ; 

 but the layers of columnar trap appear to me to be an exception to this 

 rule : they appear to have been the last that were produced in the suc- 

 cession at Bengore Head. 



We find that vertical trap dykes consist often of a series of columns 

 of four, five, six, or seven sides, exactly similar in form to those at 

 Bengore Head, but not similar in position. The Bengore Head co- 

 lumns all affect a vertical position ; those in the dykes are hori- 

 zontal. In both cases the axis of the column is at right angles 

 to the sides or surfaces between which the melted matter of the 

 trap was injected; and this law appears to be general in all cases 

 of injected trap, whether the fissures which received it were ver- 

 tical, horizontal, or sloping. Cases are often met with of trap dykes 

 where part of the dyke is vertical and a part turns into a hori- 

 zontal position. In this case, as well as in the others, the columnar 

 structure is changed in position, according to the change in direc- 

 tion of the dyke, and the axes of the columns are still at right an- 

 gles to the cooling surfaces of hard rock that existed when the fissure 

 was formed, and the melted matter poured into it. 



All the varieties of trap, the most dense heavy black basalt, 

 and the most porous white lava; the hard rough trachyte and the 

 soft red bole are composed for the most part of the same elementary sub- 

 stances. Since this is the case, they must have been produced under 

 different conditions. Some of the flows were erupted in deep water 

 in the bottom of an ocean, some poured out in the atmosphere, and some 

 into fissures in cold hard rocks, which were split; up or dislocated by 

 subterraneous expansive power, those fissures affording a facility for 

 the melted lava to penetrate them, and there harden into rock as hard 

 as that which encloses it. 



It appears to me that the trap rocks of Antrim have been produced 

 under the following conditions : — The black vesicular tabular trap which 

 it is believed lies next over the chalk on the north coast, about Bengore 

 Head, was erupted in a deep ocean, and spread out in liquid masses over 

 the rocky bottom of that ocean, generating steam which produced the 

 cells in the mass. The water cooled it quickly without giving time for 

 crystallization ; hence its dull rough fracture. In this manner I suppose 

 the black beds Nos. 1, 2, 3, on the section (PL XXIV.) were produced, 

 which amount to about 80 feet over the water. 



I believe the ochre or red bole was volcanic ashes thrown up in the 

 eruption, and disseminated in the water, making literally a red sea. 

 When the energy of the first burst of volcanic action was partly spent, 

 there came a time of rest, and in a calm, part of the red sediment was 

 deposited, making a layer of ochre. Over this again was erupted and 

 spread out another layer of the fused matter of the black rock ; then in 

 a second calm, another deposit of red ashes as before. In this manner 

 was produced the alternations of black rock and red ochre layers, as we 

 see them about Bengore Head (PI. XXIV.), numbered on the section 4, 

 5, 6. No. 7 is a bed of red ochre 22 feet thick, produced in the same 



