SECTION OF CLIFFS NEAR GIANT'S CAUSEWAY. 415 (on the south of the great valley of denudation, through which the Mayola flows,) the chalk appears underneath the basalt, and there it attains its greatest elevation, being quarried at the very considerable height of 1460 feet above the level of the The deposit is however thin, and the strata much split. sea. The following section of the cliffs near the Causeway, shows the order of superposition of the different trap rocks. 7. Columnar basalt, the upper range of pillars at Ben gore head, 54 8. Irregularly prismatic basalt. In this bed the wacke 54 44 The tabular basalt is the prevailing rock of the trap district, occupying at least nine-tenths of its whole area. It is disposed in beds of considerable thickness. The strata of columnar basalt seem to occur almost exclusively towards the northern boundary of the basaltic area. The pillars are composed of the most compact and homogeneous variety of basalt, containing a small quantity of steatite occasionally imbedded in its mass, and possessing the property of being more or less sonorous when struck with the hammer. These columns appear in a great many localities besides that of the Giant's Causeway. The following remarks on basaltic formations by the Rev. W. Conybeare form a note to his valuable paper on the North-East of Ireland in the 3d volume of the Geological Transactions. While describing the striking appearances presented by Kenbaan cliffs, I cannot forbear to declare the conviction which this spot first impressed upon my mind, and to express my full assent to the arguments of those who maintain the igneous origin of such formations. I would observe then that this formation is distinguished by characters so directly opposed to those which all rocks undoubtedly of aqueous origin possess, that no hypothesis which ascribes both to a common origin, can be otherwise than contradictory, and at variance with itself. For, 1. Of all other formations, the least ancient are the least elevated; but this the most recent of all, yet rivals the primitive mountains in height. 2. Of all other formations, the degree of consolidation decreases together with their age, their texture passing from crystalline through the several gradations of sub-crystalline, compact, coarse, and lastly earthy; while in this formation, even where it rests on chalk, the crystalline texture of the oldest rocks frequently recurs. 3. Whin dykes, which are indisputably connected with this formation, differ from all other mineral veins, in the circumstance of their traversing all rocks indifferently; while of other veins, particular classes are exclusively associated with particular IGNEOUS ORIGIN OF BASALT PROVED. 417 rocks. Such being the negative evidence against the Neptunian hypothesis, I proceed to that which is positive in favour of the Volcanists; as, 1. The identity of chemical composition in basalt and lava. 2. The constant occurrence of trap rocks in volcanic districts. 3. The confession of the Wernerians themselves, that the basalt of Auvergne is of igneous origin. 4. The testimony of those best acquainted with districts still exhibiting active volcanoes. Such persons as Dolomieu and Spallanzani, have uniformly maintained the igneous origin of basalt, while those who have contended against it, have generally been unacquainted with countries of this description. Having thus alluded to, rather than stated, some of the general arguments on which this question appears to me to depend, I return to Kenbaan, where the basalt is seen extending from beneath, as well as overlying, the great mass of chalk, which has at one extremity assumed such a curvature as would naturally result from lateral pressure; and at the other is rent, and shattered in the most extraordinary manner, the basaltic matter insinuating itself into the fissures, and often converting by its contact, chalk into granular marble, while fragments of the chalk of all sizes appear to have been forced upwards, and imbedded in the basaltic rock, having suffered in their superficial parts, where the basalt touches them, a most remarkable change. It seems impossible to conceive appearances more utterly irreconcilable with the hypothesis, that the basalt was deposited regularly above the chalk from a state of aqueous solution. On the other hand were we to imagine, a priori, the phenomena which would probably result from the eruption of a current of ignited lava from beneath the chalk, and its subsequent diffusion over the upper surface of the chalk, while the whole was submerged beneath the sea, and under a considerable pressure, they would exactly accord with those which may actually be observed at Kenbaan. To the same purpose the change effected by the whin dykes of this district on the rocks they traverse might be cited. Thus we have instances; 1. Of the conversion of old red sandstone into hornstone.-Geol. Trans. vol. III. p. 201. 2. Of the conversion of the slate clay of the coal measures into flinty slate, and of the reduction of the coal itself to cinders.— Ibid. pp. 205, 206. 3. Probably also of the conversion of the slate-clay of the lias formation into flinty slate.-p. 213. 4. Of the conversion of chalk in several places into granular marble. pp. 172, 173. Hence if it be allowable to speculate on subjects so remote from actual observation, I would infer that the hypothesis which ascribes the formation of the floetz trap rocks to submarine volcanoes, which were active at a very remote period before the seas and continents had assumed their present relative level, is both in itself more consistent, and in its application to the actual phenomena more satisfactory than any other. It is evident that the basaltic mass of Ulster was accumulated antecedently to the last great convulsion which has modified the surface of our globe, excavating its valleys, and constituting its alluvial deposits.-Geol. Trans. vol. III. p. 208. Professor Buckland in his general outlines of the structure of the Alps, informs us that there are no traces whatever either of trap rocks, or whin dykes, in the Alps of Savoy, Switzerland, or Tyrol, along the entire north side of the great primitive ridge from Mont Blanc to Presburg. But on the south side of it in Tyrol, they occur in considerable force at the Val di Fassa on the east of Botzen, under circumstances of singular resemblance to the trap rocks and whin dykes of Scotland, and the north of Ireland. See page 424. An excellent mineralogical account of this district has been given by Professor Brocchi of Milan. BASALTIC ERUPTIONS IN THE ALPS. 419 The trap protrudes itself through primitive rocks, new red sandstone, and Alpine limestones, both in the form of small dykes and irregular masses; the latter swell into mountains of great elevation at the upper extremity of the Val di Fassa, above Vigo; and in the Sieger Alp close adjoining. They abound in well crystallized minerals, chiefly of the zeolite family, which bear a strong resemblance to those of the neighbourhood of Glasgow. In both countries also the rock itself comes much into contact with, and cuts through strata of the new red sandstone formation. A similar mass of trap occurs also cutting the Alpine limestone of Monte Baldo on the Lago di Garda, where it is remarkable for containing veins and nodules of the green earth of Verona, a substance which probably derives its colour, if not origin, from the decomposition of pyroxene (augite). Not far south from Fassa on the border of the plain of Lombardy, is a still more extensive formation of trap, which occupies large tracts in the Vicentino, the Monti Berici, and Euganean hills.See page 95. In these districts, basaltic dykes cut through rocks of all ages, from the mica slate of Recoaro to the calcaire grossier (crag limestone) of Monte Bolca, and the Monti Berici; and amorphous masses of trap protrude themselves into and through these same formations, so as to appear, in different points, lying under, over, and alternating with them all. In the Euganean hills, the trap has been said to contain marine shells; and hence an argument has been taken against its igneous origin. But |