MEISNER.

was conceived that this crystalline structure would have been obliterated by heat ;* and even Dolomieu was led by this consideration to admit the Wernerian doctrine with respect to greenstone. At that time the experiments of Sir J. Hall, Watt, and others, had not induced Geologists to admit that these crystals might have been the very result, under certain circumstances,† of the process, which was at first imagined to be incompatible with their existence.

Near Cassel is a lofty ridge of mountains called the Habichwald, which, including the Dornberg and other hills connected with it in character and position, forms a large square of about four German or twenty English miles, to the west of that.city.

It consists principally of beds of tuff, associated with scoriform lava and compact basalt, the whole resting on a bed of brown coal, or, where that is wanting, upon limestone.

The tuff consists of a congeries of highly. cellular and vitreous varieties of lava, which seem to denote a recent origin compared with that of the rocks in the neighbourhood. Its structure is well displayed near the pleasuregrounds of the Elector, above the palace of Wilhelmshöhe.‡

The hills above particularized may serve to illustrate the general structure of those numerous basaltic cones which lie scattered for a considerable distance on all sides of the Seven mountains, the Westerwald, and the Vogelsgebirge, outliers, at it were, of that great volcanic formation, which, in the latter places, covers the whole face of the country.

Other examples of the same kind are furnished by the neighbourhood of Frankfort and Hanau, where small wedgeshaped prominences of compact basalt, gradually becoming

* See Daubuisson on Basalt.

+ See my Fourth Lecture.

See Raspe's account of some German Volcanos, London, 1776.

NEAR FRANKFORT.

cellular near the surface, appear to have been thrust through the midst of the sandstone formation.

At Steinheim near Hanau the cells of the basalt are occupied by a variety of sparry iron ore, called by Haussman sphæro-siderite, which forms a number of spheres varying from a line to an inch and a half in diameter, more generally quite compact, but sometimes hollow, and containing within a nucleus of a yellowish or ochreous matter, effervescing feebly with acids, and yielding with difficulty to the knife. I observed some circumstances relative to this mineral and the rock containing it, which seem to deserve a brief notice, One was the occurrence of a sort of vein of cellular basalt passing through the substance of the compact, like the coarse-grained granite which we sometimes see penetrating a fine-grained variety. In the midst of the cellular portion was a cavity filled with the spathose iron above noticed, as in the sketch underneath, where

a a are the portions of compact basalt; B B those which are cellular; C the cavity with sphæro-siderite coating its walls.

Another circumstance which I remarked, was, that the external surface of the spheres had in some cases a covering of white calcareous matter, in a powdery form, sprinkled over it.

The third observation I made indicated still more strongly that the whole had been in fusion, namely, the spheres being sometimes found flattened at the points where they appeared to have been in contact with the lava, an effect arising in all

GENERAL REMARKS.

probability from the contraction that took place in the surrounding parts whilst undergoing cooling.

Semi-opal also occurs between the interstices of this same rock, and I have even seen specks of noble opal disseminated through its substance.

The basalt both near Frankfort and near Hanau shews a remarkable tendency to concentric arrangement, and as it contains much iron, which becomes readily oxidized, the external layers decompose and peel off, leaving only a small nucleus of compact basalt retaining its original characters. In some cases the cellular and compact basalt occur intermixed, but in others the former is confined to the superficial portions. The cells however in these instances do not form that sort of net-work which is usual to them, but constitute long cylindrical tubes distinct from each other, just such as would be occasioned by bubbles of gas forcing their way upwards through a soft pulpy substance.

Enough has already been said respecting the general structure of the Westerwald and the Vogelsgebirge, which appear to be composed of extensive plateaus and cones of basalt, covering the rock of the country, but never alternating with it. Nor is there much to be learnt with regard to the manner of their formation by an examination of the districts themselves; and a Wernerian, if he were to overlook or explain away the fact of cellular products occurring amongst them, might easily persuade himself that the whole was the result of aqueous deposition, the evidence of that return of the waters, which is supposed to have given birth to the newest floetz trap formation of his master.

When however we turn to the dykes of basaltic matter (as they may be called) which are scattered all around, we can hardly help imagining that these more extensive formations of the same rock have in reality been produced in the same manner, that the more elevated masses, which are ge

NEAR HEIDelburg.

nerally most compact, were first thrown up by the agency of a volcano, and that the cellular matters being subsequently ejected, arranged themselves around them in successive strata. The volcanic operations, taking place with the greatest intensity round the area now occupied by the Vogelsgebirge and the other basaltic groupes, would cover completely with their products the surface of the subjacent rock; whilst at a greater distance from the sphere of their activity, isolated cones of basaltic matter would be occasionally thrown up, as at Eisenach, at Cassel, in the neighbourhood of Frankfort, and on the west bank of the Rhine.

Besides the volcanos above enumerated, there occur near the borders of the Rhine, but higher up the stream, other rocks which are said to have a similar origin.

In the Odenwald, a group of hills in the neighbourhood of Heidelburg, from the midst of the new red sandstone, rise some eminences in which basalt is associated with augite rock (dolerite of Brongniart), and contains nepheline (Katzenbuckel), mica, mesotype, olivine, and titaniferous iron ore. It is probably analogous to the Hessian basalts. The augite rock is seen in situ at Gaffstein, the basalt containing olivine at Pechsteinkopf and Durkheim.*

Near Freyburg in the Brisgaw is the group of the Kaiserstuhl, of which Dr. Boué has given an account in his memoir on the South-west of France. It appears from his report to be a mass of augite rock with excess of felspar (dolerite felspathique) thrown up from the midst of the plastic clay.

The highest mountain in the groupe is the Kaiserstuhl, which rises to the height of about 1120 feet above the river, and this with the other eminences composed of the same materials are ranged in an elliptical form round a valley.

These rocks offer no trace of craters or streams of lava,

* See Leonhard Taschenbuch for 1822.

+ Annales des Sciences Naturelles, August, 1824.

BRISGAW.

but are associated, especially near the surface, with cellular lava, containing calcareous spar and mesotype. Hyalite occurs in the cavities, and incrusting the surfaces of the rock. Tufaceous matters are not common, but occur along the Rhine at Breisach, where they seem, like those near Eisenach, to be contemporaneous with the augite rock.

I am also assured that the rock of Kaiserstuhl is partially covered with a calcareous deposit, the only instance I believe among the Rhine volcanos in which this occurs.

Saussure,+ who visited this groupe in 1794, and appears to have been somewhat swayed by the authority of Werner, is nevertheless compelled to acknowledge the volcanic origin of the rocks about Limburg, which are in part penetrated with oval cells, in great measure void, and of the tuff about Echardberg, which contain fragments of scoriform lava. The origin of the basalt itself he considers doubtful, but there are few at the present day who will concur with him in that opinion, considering how intimate appears to be the connection between the porous and compact rocks in this locality. Upon the whole the groupe of the Kaiserstuhl may be set down as belonging to the same æra as the basalt of the Westerwald, and the trachyte of the Seven Mountains.

A few miles to the north of the Lake of Constance is the commencement of another series of basaltic and porphyritic cones, first seen at Hohentwiel and in several detached hills contiguous, which rise from the midst of the (Jura?) limestone formation.

They consist in part of clinkstone, and in part of basalt, accompanied with tuff containing fragments of trap rock (always compact) as well as of gneiss, limestone, quartz, &c. all cemented by a wacke-like paste of a ochrey colour.

In the hill of Magdeburg, a passage is said to exist be

+ Journal de Physique, vol. 44.

F