240 Froceedhuj.'s of the Royal Irkh Academy. 



If the breadth, of the joint remained quite ■unifonn and unacted 

 upon, the only action of the walls on the fluid mass would be that of 

 reducing the temperature (wherever that temperature had not already 

 attained that of the basalt). The differences of temperature thus pro- 

 duced should give rise to banding more or less defined and regular. 

 ^VTiere, however, the section of joint presented an enlargement or a 

 contraction, there not merely banding, but the formation of whorls, 

 must have to some extent resulted, the fluid mass continuing in move- 

 ment. "Where the enlargement or contraction was very marked and 

 sudden, then such whorls would have been most likely to be formed, and 

 should, if formed, present a direction parallel to the intersection of 

 the joint with the direction of the bedding. As, however, banding is 

 assumed to have been going on simultaneously, the whorl should pre- 

 sent, when solidified, lines indicative of that banding, the final cooling 

 and solidification giving rise to the radial jointing. Those whorls 

 would have tended to increase in diameter or number of bands with 

 the persistence of the flow, and, moreover, would tend to uplift the 

 strata or beds traversed by the joint, and at least to open the planes 

 of bedding limiting such beds. Injection of the fluid basalt into 

 those planes of bedding would thus be set up, and might continue, 

 according to the conditions of pressure and of fluidity. There would 

 thus be established a connexion between the dykes, their direction, 

 the whorls, and the intercalation of beds of basalt between tfie pre- 

 viously existing beds traversed by the joint. Such a connexion is to 

 some extent implied by the description given by Eegnault of the Rhine 

 basalt mass of Langenberg. It really exists in the case which I am 

 discussing, since the direction of the cylindrical mass is that of the 

 coast line, itself dependent on the jointing of the country. 



There is a further and very interesting consideration in connexion 

 with this whorl form, if it be admitted to have been formed as sug- 

 gested. It is that of the temperature and of the correlative depth 

 from surface at which the whorl was formed. Admitting for the fluid 

 basalt a very slow movement, it is quite clear that the formation of the 

 whorl would be equally slow, and even might be slower, and that the 

 cooling should have been veiy slow indeed to allow of the continuous 

 formation of one of those cyLLndrical masses. Such a condition of 

 temperature would best be satisfied by the action having taken place 

 at a relatively great depth, that is to say, a depth corresponding to 

 that at which basalt melts, which may be taken at 700° or 800° C. at 

 least. Or, by supposing that the fluid mass had remained in the 

 joint such a length of time as to communicate to the sides an amount 

 of heat sufiicient to allow of a continuous movement in the mass with- 

 out rapid cooling and solidification ensuing. In any case a certain 

 depth from surface may be assumed as ha^-ing been one of the condi- 

 tions attending the formation of the cyhncbical mass in question. 



That the coast of Antrim has at some time been under the present 

 level is sufiiciently proved by the geological strata existing there. 



