Physique, vol. li. p. 162.), throws light at once, 1st, Upon the large crystals which are isolated and separated from the microscopic crystals entangled in the mass; 2dly, On the mutual passages of some rocks, superimposed the one upon the other; 3dly, On the subordinate beds, which are of the sanie nature as one of the elements of the amorphous mass. All these phenomena are produced, if we may so speak, by internal development; by variation in the constituent parts of a heterogeneous mass. Crystalline molecules, invisible to the eye, occur enlarged and disengaged from the compact tissue of the paste; by their assemblage and mixture with new substances, they insensibly become intercalated beds of considerable thickness; and not unfrequently they even become new rocks.

It is the intercalated beds which especially merit the greatest attention. (Leonhard, Kopp and Gartner, Propad. der Miner., p. 158.) When two formations succeed each other immediately, it happens that the beds of the one begin at first to alternate with the beds of the other, until (after these precursors of a great change) the newest formation shows itself without any mixture of subordinate beds. (Buch, Geogn. Beob, vol. i. p. 104. 156.; Humboldt, Rel. Hist, vol. ii, p. 140.) The progressive developments of the elements of a rock, may, consequently, have a great degree of influence upon the relative position of the mineral masses. Their effects belong to the province of geology; but, in order to discover and appreciate them, the observer must call to his assistance oryctognosy.

In exposing the intimate relations by which we often see the phenomena of composition connected with those of relative situation, it has not been my intention to speak of the purely oryctognostic method, which considers rocks áccording to the analogy of their composition alone. (Journal des Mines, vol. xxxiv. No. 199.) In the classifications of this method, abstraction is made of every idea of superposition; but they do not the less give rise to interesting observations regarding the constant assemblage of certain minerals. A purely oryctognostic classification, multiplies the names of rocks more than is required by geognosy, when occupied with superposition alone. According to the changes which the mixed rocks undergo, a stratum of great extent and thickness may contain (we must repeat it here) parts VOL. X. NO. 19. JAN. 1824.

D

to which the oryctognost, who classes rocks according to their composition, would give entirely different denominations. These remarks have not escaped the learned author of the Classification Mineralogique des Roches; they must have presented themselves to an experienced geognost, who has so successfully investigated the superposition of the deposites of which he has treated. "We must not confound," says M. Brongniart, in his late Memoir on the position of the Ophiolites, "the relative positions, the orders of superposition of the deposites and of the rocks which compose them, with purely mineralogical descriptions. The neglect of making the proper distinction in this case, would necessarily be productive of confusion in the science, and would retard its progress." The arrangement which we give at the end of this article, is by no means what is called a classification of rocks; there will not even be found united, under the title of particular sections (as in the old geognostical method of Werner, or in the excellent Traité de Geognosie of M. D'Aubuisson), all the primitive formations of granite, nor all the secondary formations of sandstone and limestone. It has been attempted, on the contrary, to place each rock as it occurs in nature, according to the order of its superposition or of its respective age. The different formations of granite are separted by gneisses, mica-slates, black-limestones and greywackes. In the transition rocks, we have separated the forma tions of porphyries and syenites of Mexico and Peru, which are anterior to the grey-wacke, and to the limestone with orthoceratites, from the much more recent formation of the zircon-porphyries and syenites of Scandinavia. In the secondary rocks, we have separated the oolitic sandstone of Nebra, which is posterior to the alpine limestone or zechstein, from the red-sandstone, which belongs to the same formation with the secondary porphyry and amygdaloid. According to the principle which we follow, the same names of rocks occur several times in the same table. Anthracitic mica-slate is separated, by a great number of older formations, from the mica-slate anterior to the primitive clayslate.

Instead of a classification of granitic, schistose, calcareous and arenaceous rocks, it has been my object to present a sketch of the geognostical structure of the globe; a table in which the

superimposed rocks succeed each other, from below upwards, as in those ideal sections which I designed in 1804, for the benefit of the Mexican School of Mines, and of which many copies have been distributed since my return to Europe. (Bosquejo de una Pasigrafia geognostica, con tablas que ensenan la estratificacion y el paiallelismo de las rocas en ambos Continentes, para el uso del Real Seminario de Mineria de Mexico.) These Pasigraphic tables united to my own observations made in both Americas what had at that period been known with precision regarding the relative position of the primitive, intermediary and secondary rocks in the Old Continent. They presented, together with the type which might be considered as the most general, the secondary types, that is to say, the beds which I have named parallel. This same method has been followed in the work which I now publish. My parallel formations are geognostical equivalents; they are rocks which represent each other. (See the Traité de Geologie de M. d' Aubuisson, vol. ii. p. 255.) In England, and on the opposite Continent, there does not exist an iden tity of all the formations: there exist equivalents or parallel formations. That of our coal situated between the transition masses and the red-sandstone, the position of the rock-salt which occurs on the Continent in the alpine limestone, and the position of our oolites in the Nebra sandstone and Jura limestone, may guide the geognost in the approximation of remote formations. In England, we observe the coals placed upon transition formations; for example, upon the mountain-limestone of Derbyshire and of South Wales, and upon the transition sandstone, or old redsandstone of Herefordshire. I have thought that I recognised in the magnesian-limestone the red marl, the lias and white oolites of Bath, the united formations of the alpine limestone, of the oolitic sandstone and Jura limestone. In comparing the formations of countries more or less distant from each other, those of England and of France, for instance, of Mexico and Hungary, of the secondary basin of Santa Fe de Bogota and of Thuringia, we must not think of opposing to each individual rock a parallel one; it must be recollected, that a single formation may represent several others. It is according to this principle that beds of clay, lying beneath the chalk, may, in France,

be separated in the most distinct manner from the oolitic limes. stone beds; while in Switzerland, in Germany, and in South America, they have for equivalents beds of marls subordinate to the Jura limestone. The gypsums, which, in one district, are sometimes only intercalated beds in the alpine limestone or oolitic sandstone, in another district, assume all the appearance of independent formations, and occur interposed between the alpine limestone and the oolitic sandstone, between this sandstone and the muschelkalk. The learned Oxford Professor, Mr Buckland, whose extensive researches have been equally useful to the geognosts of England and of the Continent, has lately published a table of parallel formations, or, as he calls them, equivalents of rocks, which only extends from the 44th to the 54th degree of north latitude, but which merits the greatest attention. (On the Structure of the Alps, and their relation with the rocks of England, 1821.)

As in the history of ancient nations, it is easier to verify the series of events in each country, than to determine their mutual coincidence; so also more accuracy can be attained in estima ting the superposition of formations in isolated regions, than in determining the relative age or parallelism of formations which belong to different systems of rocks. Even in countries which are not widely separated, in France, in Switzerland, and in Germany, it is not easy to fix the relative antiquity of the muschelkalk, of the molasse of Argovie, and of the quadersandstein of the Hartz; because rocks of general occurrence are here most commonly wanting, which, according to the happy expression of of M. de Grüner, serve as a geognostical horizon, and with which we might compare the three formations in question. When rocks are not in immediate contact, we can only judge of their parallelism by the relations of age existing between them and other formations, by which they are united.

These inquiries of comparative geognosy, will long occupy the sagacity of observers; and it is not surprising that those who set out with the idea of retracing each formation in all the individuality of its relative position, interior structure and subordinate beds, should finish with utterly denying all analogy of superposition. I had the advantage of visiting, previous to my journey to the Equator, a great part of Germany, of France, of

Switzerland, of England, of Italy, of Poland, and of Spain. During these excursions, my attention was particularly directed to the relative position of formations, a phenomenon which I calculated upon discussing in a special work. On my arrival in South America, and while at first traversing in different directions the vast deposites which stretch from the maritime chain of Venezuela to the basin of the Amazon, I was singularly struck with the conformity of position which the two Continents present. (See my first sketch of a Geological Table of Equinoctial America, in the Journal de Phys., vol. liii, p. 38.) Subsequent observations, which included the Cordilleras of Mexico, of New Grenada, of Quito, and of Peru, from the 21st degree of north latitude to the 12th degree of south latitude, have confirmed these first perceptions. But in speaking of analogies which are observed in the relative position of rocks, and of the uniformity of those laws which reveal to us the order of Nature, I might adduce a testimony otherwise of more weight than mine, that of the great geognost whose works have thrown the greatest light upon the structure of our globe. M. Leopold de Buch has pushed his researches from the Archipelago of the Canary Isles to beyond the Polar Circle to the 71st degree of latitude. He has discovered new formations situated between others already known; and, in the primitive as in the transition deposites, in the secondary as well as in the volcanic, he has been struck with the great features by which the table of formations is characterized in the most distant regions.

(To be continued.)

ART. VII. Description of a simple, cheap, and accurate Method of experimenting on small quantities of Gases, by means of Bent Tubes. By Mr WILLIAM KERR.

THE greatest obstacles to experimental research that have hi

therto presented themselves to young chemists, and even to proficients in the science, are the expence of the requisite apparatus, and the want of room to contain them. This has especially been the case with respect to apparatus for experimenting on