sides, by any means exclude the uniformity or concordance of position; it rather excludes the oryctognostic passage of two superimposed formations. The transition districts have very often the same direction and the same inclination as the primitive ones; and yet, whatever approximation there may be between their origin, we are not the less warranted to consider the anthracitic mica-slate or the grey-wacke, alternating with porphyry, as two formations independent of the primitive granites and gneisses which they cover. The conformity of position is in no way incompatible with the independence of formations, that is to say, it does not prevent the right which one has of regarding a rock as a distinct formation. It is because the independent formations are placed indifferently on all the older rocks, (the chalk upon the granite, the red sandstone upon the primitive mica-slate), that the assemblage of a great number of observations made upon very distant points, becomes eminently useful in the determination of the relative age of rocks. In order to determine that the zircon-sienite is a transition rock, it must have been seen resting upon formations posterior to the black limestone with orthoceratites. Observations made upon the porphyries and sienites of Hungary by M. Beudant, one of the most distinguished geologists of the present times, may throw much light upon the formations of the Mexican Andes. It is thus that a new vegetable discovered in India, elucidated the natural affinity between two families of plants belonging to Equinoctial America.

The order which has been followed in the table of formations, is that of the situation and relative position of rocks. I do not pretend that this position is observed in all the countries of the globe; I merely point it out such as it has appeared the most probable, after the comparison of a great number of facts which I have collected. It is by the idea of the relative age of formation, that I have been guided in this work, imperfect as it still is. I had begun it long before my journey to the Cordilleras of the New Continent, from the year 1792, when, on leaving the Freyberg School, I was appointed to the direction of the Mines in the mountains of the Fichtelgebirge. The same rock may vary in composition, integrant parts may have been abstracted, and new substances may occur disseminated, without the rock's changing its denomination in the eyes of the geognost who is engaged with the superposition of formations. Under the equator, as in the north of Europe, strata of a true transition syenite lose their hornblende, without the mass becoming another rock.

The granites of the banks of the Orinoco sometimes assume hornblende as an integral part, and yet do not cease to be primitive granite, although this may not be of the first or oldest formation. These facts have been observed by all practical geologists. The essential character of the identity of an independent formation is its relative position, the place which it occupies in the general series of formations. (See the classical Memoir of M. de Buch, Ueber den Begriff einer, in the Mag. der Naturf., 1810, p. 128-133.) It is on this account that an isolated fragment, a specimen of rock found in a collection, cannot be determined geognostically, that is to say, it cannot be referred with certainty to a particular formation, constituting one of the numerous beds of which the crust of our planet is composed. The presence of chiastolite, the accumulation of carbon or nodules of compact limestone in the clay-slates, nigrine and epidote in the syenites, (alternating with granite and porphyries), conglomerates or pudding-stones, having a basis of anthracitic mica-slate, may, without doubt, be characteristic of transition formations; in the same way as, according to the useful labours of M. Brongniart, petrifactions of shells, in a good state of preservation, sometimes indicate directly such or such a bed of tertiary deposits. But these cases, where we are guided by disseminated substances or by characters purely geological, comprehend but a small number of rocks of a recent origin, and observations of this kind often lead only to negative facts. The characters taken from the colour, from the grain, and from small veins of carbonate of lime, which traverse calcareous rocks; those which are furnished by the fissility and silky lustre of clay-slate, the aspect and undulations more or less marked of the scales of mica in mica-slate; and, lastly, the size and colouring of the crystals of felspar in the granites of different formations, may, like all that is connected simply with the physiognomy of minerals, lead the most expert observer into error. The white and black tints undoubtedly in most instances distinguish the primitive and transition limestones; the Jura formation, especially in its upper beds, is also without doubt generally divided into thin whitish beds, having a dull, even, or conchoidal fracture, with very flat cavities: but in the mountains of transition limestone there are isolated masses which, in colour and texture, resemble the oryctognostic characters of the Jura limestone; and to the south of the Alps there are hills belonging to tertiary deposits, where we find rocks analogous to the slaty and dull Jura limestone (in as far as regards appearance), in formations

placed above the chalk, and which resemble the limestone used for lithographic purposes. Were names taken from their oryctognostical characters alone to be preferred in distinguishing formations, the different strata of the same compound rock having a considerable thickness, and extended to a great length in a particular direction, would often seem to belong to different rocks, according to the points at which specimens were taken. Consequently we can only determine geognostically in collections, suites of rocks of which the mutual superposition is known.

In announcing these ideas regarding the sense which should be attached to the words independent formations, when treating of the order of their position, we are very far from undervaluing the eminent services which the most rigorous oryctognostic examination, the minute investigation of the composition of rocks, have rendered to modern geognosy, and especially to the knowledge of the relative position of formations. Although, according to the discoveries of M. Haüy, regarding the intimate nature of inorganic and crystallized substances, there does not exist, properly speaking, a passage or transition of one mineral species to another; (Cordier, sur les Roches volcan., p. 33, and Berzelius, Nouv. Syst. de Mineral, p. 119) the passages of masses or pastes of rocks, are not limited to formations which are commonly distinguished by the name of Compound Rocks. Those which are thought simple, for example, the transition or secondary limestones, are partly amorphous varieties of mineral species, of which there exists a crystallized type, partly of aggregates of clay, carbon, &c., which cannot be submitted to any fixed determination. It is upon the variable proportions of these heterogeneous mixtures, that the passage of marly limestones to other schistose formations is founded. (Haüy, Tableau comparatif de la Cristallographie, p. 27—30). All the amorphous pastes of rocks, however homogenous they appear at first sight, the bases of porphyries and euphotides (serpentines), as well as those problematical black masses which constitute the basanite (basalt) of the ancients, and which are not all greenstones surcharged with hornblende, are susceptible of being subjected to mechanical analysis. M. Cordier has applied this analysis in an ingenious manner to the diabases, dolerites, and other more recent volcanic productions. The most apparently minute oryctognostic examination, cannot be indifferent to the geognost who examines the age of formations. It is by this examination that we are enabled to form a just idea of the progressive manner in which, by internal develop

ment, that is to say, by a very gradual change in the proportions of the elements of the mass, the passage is made from one rock to a neighbouring. The transition slates, whose structure appears at first so different from that of the granites or porphyries, present to the attentive observer striking examples of insensible passages to granular rocks of porphyritic or granitic nature. These slates become at first greenish and harder. In proportion as the amorphous paste receives hornblende, it passes into those hornblendic traps which in former times were confounded with basalt. In other cases, the mica, which is at first concealed in the amorphous paste, becomes developed, and separates into distinct and clearly crystallized spangles; at the same time, the felspar and quartz become visible; and the mass assumes a granular aspect, with very elongated grains: this is a true transition gneiss. By degrees, the grains lose their common direction; the crystals arrange themselves around many centres; the rock becomes a transition granite or syenite. In other cases, the quartz alone is developed; it augments, and becomes rounded into nodules, and the slate passes to the best characterized grey-wackes. By these certain signs, geognosts, to whom the appearances of nature have become familiar by long examination, become aware beforehand of the proximity of granular, granitic, and arenaceous rocks. Analogous passages of primitive micaslate to a porphyritic rock, and the return of this rock to gneiss, are observed in the eastern parts of Switzerland. (See the luminous developments given by M. de Raumer, Fragments, p. 10, and 47; M. Leopold de Buch, in his Voyage de Glaris à Chiavenna, fait en 1803, and inserted in the Magaz. der Berl. Naturf., vol. iii. p. 115). But these passages are not always insensible and progressive; the rocks often also succeed each other quickly, and in a very abrupt manner; often (for example, at Mexico, between Guanaxuato and Ovexeras), the limits between the slates, the porphyries and syenites, are as distinct as the limits between the porphyries and limestones; but even in this case, geognostical relations with the superimposed rocks are indicated by additional heterogeneous beds. It is thus that the transition granite of the syenitic formation presents beds of basanite, by becoming charged with hornblende it is thus, also, that these same granites sometimes pass to euphotide. (Buch, Voyages en Norwege, vol. i. p. 138, vol. ii. p. 83).

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There results from these considerations, that the mechanical. analysis of amorphous pastes, by means of demi-triturations, and washings, (an analysis of which, M. Fleuriau de Bellevue

made the first attempt, that was crowned with success: Journ. de 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 same 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 Gærtner, Propæd. 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 according to the analogy of their composition alone. (Journal des Mines, vol. xxxiv., No. 199). În 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 to which the oryctognost, who classes rocks according to their composition, would give entirely different denominations.