The strata in this space consist, in general, of sand-stone, coal, lime-stone or marble, iron-stone, and marl or argillaceous strata, with strata of analogous bodies, and the various compositions of these. But what is to the present purpose is this, that, through all this space, there are interspersed immense quantities of whinstone; a body which is to be distinguished as very different from lava; and now the disposition of this whin-stone is to be considered.
Sometimes it is found in an irregular mass or mountain, as Mr Cronstedt has properly observed; but he has also said, that this is not the case in general. His words are: "It is oftener found in form of veins in mountains of another kind, running commonly in a serpentine manner, contrary or across to the direction of the rock itself."
The origin of this form, in which the trap or whin-stone appears, is most evident to inspection, when we consider that this solid body had been in a fluid state, and introduced, in that state, among strata, which preserved their proper form. The strata appear to have been broken, and the two correspondent parts of those strata are separated to admit the flowing mass of whin-stone.
A fine example of this kind may be seen upon the south side of the Earn, on the road to Crief. It is twenty-four yards wide, stands perpendicular, and appears many feet above the surface of the ground. It runs from that eastward, and would seem to be the same with that which crosses the river Tay, in forming Campsy-lin above Stanley, as a lesser one of the same kind does below it. I have seen it at Lednoc upon the Ammon, where it forms a cascade in that river, about five or six miles west of Campsy-lin. It appears to run from the Tay east through Strathmore, so that it may be considered as having been traced for twenty or thirty miles, and westwards to Drummond castle, perhaps much farther.
Two small veins of the same kind, only two or three feet wide, may be seen in the bed of the Water of Leith, traversing the horizontal strata, the one is above St Bernard's well, the other immediately below it. But, more particularly, in the shire of Ayr, to the north of Irvine, there are to be seen upon the coast, between that and Scarmorly, in the space of about twenty miles, more than twenty or thirty such dykes (as they are called) of whin-stone. Some of them are of a great thickness; and, in some places, there is perceived a short one, running at right angles, and communicating with other two that run parallel.
There is in this country, and in Derbyshire[14], another regular appearance of this stone, which Cronstedt has not mentioned. In this case, the strata are not broken in order to have the whin-stone introduced, they are separated, and the whin-stone is interjected in form of strata, having various degrees of regularity, and being of different thickness. On the south side of Edinburgh, I have seen, in little more than the space of a mile from east to west, nine or ten masses of whin-stone interjected among the strata. These masses of whin-stone are from three or four to an hundred feet thick, running parallel in planes inclined to the horizon, and forming with it an angle of about twenty or thirty degrees, as may be seen at all times in the hill of Salisbury Craggs.
Footnote 14:[ (return) ] See Mr Whitehurst's Theory of the Earth.
Having thus described these masses, which have flowed by means of heat among the strata of the globe, strata which had been formed by subsidence at the bottom of the sea, it will now be proper to examine the difference that subsists between these subterraneous lavas, as they may be termed, and the analogous bodies which are proper lavas, in having issued out of a volcano.[15]
Footnote 15:[ (return) ] The Chevalier de Dolomieu, in his accurate examination of Aetna and the Lipari islands, has very well observed the distinction of these two different species of lavas; but without seeming to know the principle upon which this essential difference depends. No bias of system, therefore, can here be supposed as perverting the Chevalier's view, in taking those observations; and these are interesting to the present theory, as corresponding perfectly with the facts from whence it has been formed. It will be proper to give the account of these in his own words.
La zéolite est très-commune dans certains laves de l'Ethna; il seroit peut-être possible d'y en rencontrer des morceaux aussi gros que ceux que fournit l'isle de Ferroé. Quoique cette substance semble ici appartenir aux laves, je ne dirai cependant point que toutes les zéolites soient volcaniques, ou unies à des matières volcaniques; celles que l'on trouve en Allemagne sont, dit-on, dans des circonstances différentes; mais je doit annoncer que je n'ai trouvé cette substance en Sicile, que dans les seules laves qui évidemment ont coulé dans la mer, et qui out été recouvertes par ses eaux. La zéolite des laves n'est point une déjection volcanique, ni une production du feu, ni même un matière que les laves aient enveloppée lorsqu'elles étoient fluides; elle est le résultat d'une opération et d'une combinaison postérieure, auxquelles les eaux de la mer ont concouru. Les laves qui n'ont pas été submergées, n'en contiennent jamais. J'ai trouvé ces observations si constantes, que par-tout où je rencontrois de la zéolite, j'étois sûr de trouver d'autres preuves de submersion, et partout où je voyois des laves recouvertes des dépôts de l'eau, j'étois sûr de trouver de la zéolite, et un de ces faits m'a toujours indiqué l'autre. Je me suis servi avec succès de cette observation pour diriger mes recherches, et pour connoître l'antiquité des laves. Minéralogie de Volcans, par M. Faujas de Saint-Fond. Here would appear to be the distinction of subterraneous lava, in which zeolite and calcareous spar may be found, and that which has flowed from a volcano, in which neither of these are ever observed.
There can be no doubt that these two different species of bodies have had the same origin, and that they are composed of the same materials nearly; but from the different circumstances Of their production, there is formed a character to these bodies, by which, they may be perfectly distinguished. The difference of those circumstances consists in this; the one has been emitted to the atmosphere in its fluid state the other only came to be exposed to the light in a long course of time, after it had congealed under the compression of an immense load of earth, and after certain operations, proper to the mineral regions, had been exercised upon the indurated mass. This is the cause of the difference between those erupted lavas, and our whin-stone, toad-stone, and the Swedish trap, which may be termed subterraneous lava. The visible effects of those different operations may now be mentioned.