A. Lava sheet. B. Sandstone Beds, dipping from every side towards the lava. CC. Line of Section along which Fig. 2 is supposed to be drawn.
We conceive that the phenomenon is to be explained by supposing the orifice through which the lava rose and overflowed the surface of the sedimentary strata to have been very much smaller in area than the extent of igneous rock at present visible; and that the pressure of the erupted mass on the soft beds beneath, aided perhaps by the abstraction of matter from below, caused them to incline towards the central point at a gradually increasing angle. The diagram, fig. 2, will serve further to illustrate this hypothesis. A is the neck or orifice by which the melted matter is supposed to ascend. C shews the sheet of lava after it has overspread the surface of the sandstone beds B, so as to cause them to assume their present inclination. The dotted lines represent the hypothetical extension of the igneous mass and sandstones previous to the denudation which they have suffered from the action of the waves.
Professor Geikie, in his admirable treatise on the Geology of the county[4], adopts a view on this subject which is somewhat different from that which is suggested in this paper. He considers that the whole mass is an intrusive neck of rock with perpendicular sides; and that it once filled up an orifice through the surrounding sedimentary strata, of which it is now the only remnant.
Fig. 2. Vertical Section through CC. Diagram (Fig. 1).
A. Orifice by which the lava ascended. B. Sandstone Beds. B´. Hypothetical extension of ditto. C. Sheet of lava spread over the sandstones B. C´. Hypothetical extension of ditto.
He admits that the inclination of the sandstone beds towards the igneous mass in the centre is a phenomenon that is somewhat difficult to explain, and suggests that a subsequent contraction of the column may have tended to produce such a result. To use his own words: “In the case of a solid column of felstone or basalt, the contraction of the melted mass on cooling may have had some effect in dragging down the sides of the orifice[5].”
But, apart from other objections, it is scarcely conceivable that this result should have been produced by the contraction of the column.
In his recent edition of Jukes's Manual of Geology (p. 269), in which he also refers to this instance, he states that in other cases of “necks” it is found to be an almost invariable rule, “that strata are bent down so as to dip into the neck all round its margin.” We are not aware to what other instances Prof. Geikie may allude; but on referring to his Memoir on the Geology of East Lothian, we find that he states in the cases of 'North Berwick Law' and 'Traprain' (which he compares with the igneous mass at Whitberry Point), that the beds at the base of these two necks, where exposed, dip away from them, and that at a high angle.
In support of the hypothesis which we have put forward, the following arguments may be urged: