페이지 이미지

indebted to the beautiful observations of Messrs Leopold von Buch, Brochant, and Haussmann. From the circumstance that, in the transition, granular rocks, perfectly destitute of organic remains, succeed to compact rocks, which contain these same remains, it has been concluded by geognosts of great name, that this alternation of shelly and not-shelly rocks, might extend beyond the deposites which we call primitive. It i. not been merely asked if the clay-slates, mica-slates, and gneisses, support the granites which have been considered as the oldest; the question has also been agitated, whether greywackes and black limestones with madrepores might not recur beneath those same granites. According to this idea, the primitive and transition rocks would only form a single deposite; and the first might be regarded as intercalated in a deposite posterior to the ão of organic beings, and which might penetrate to an unknown ão into the interior of the globe. I confess, that no direct observation can be as yet adduced in support of these opinions. The fragments of rocks which I have seen contained in the lithoid lavas of the volcanoes of Mexico, Quito, and Vesuvius, and which are thought to have been torn from the bowels of the earth, seem to belong to altered rocks of granite, mica-slate, syenite, and granular limestone, and not to greywackes and limestone with madrepores. We have preserved in the arrangement of rocks the great divisions known by the name of primitive, transition or intermediary and tertiary deposites. The natural limits of these four yelems of rocks are the clay-slate with glance-coal or ampelite and lydian stone, alternating with compact limestones and greywackes, the coal formation, and the formations which immediately succeed the chalk. In geognosy, as in descriptive botany, the subdivisions or small groups of families have more distinct characters than the great divisions or classes. It is the case with all the sciences; in which we rise from individuals to species, from species to genera, and from these to still higher degrees of abstraction. A method necessarily rests upon differently graduated abstractions, and the passages come more frequent in proportion as the characters are more complex. The transition or intermediary formations of Werner, which M. de Buch has first limited with the sagacity for which he is distinguished (Moll's Jahrb. 1798, b. ii. p. 254), are connected by the ampelitic clay-slates, the syenites with zircons, the granite sometimes destitute of hornblende, and the anthracitic mica-slate, with the primitive deposite; while the smallgrained greywackes and madreporous and compact limestones, connect them with the coal sandstones and limestones of the secondary desposites. Porphyries of very different formations have their principal seat among the transition rocks; but they break out, if we may so speak, in considerable masses towards the secondary deposites, where they are connected with the coal sandstone, while they penetrate into the primitive class only as subordinate rocks, and of little thickness. The progressive motion, or, if I may be allowed to use the expression, the extent of the oscillation of the serpentine and euphotide, is very different. Those diallage rocks, constituting many distinct formations, rarely covered with other rocks, stop short nearly at the lower boundary of the secondary formations; towards the bottom they penetrate into the primitive deposites to beyond the micaschist. The chalk seems to present a natural limit to the tertiary formations, which were first characterised by Messrs Cuvier and Brongniart, and justly, as deposites entirely different from the last secondary formations, described by the Freyberg School (Géogr. Miner, des Environs des Paris, p. 8 and 9.) Struck with the relations which exist between the tertiary deposites and the beds beneath the chalk, M. Brongniart has even recently proposed to designate the tertiary formations by the name of upper secondary deposites, (Sur le gisement des Ophiolithes, p. 37). Compare also the very interesting geognostical discussions contained in M. de Bonnard's Traité des Roches, p. 138,210, and 212.) The distinction of four deposites which we have successively named, and of which three are posterior to the development of organic life upon the globe, appears to me worthy of being retained, notwithstanding the passage of some formations to others of a very different character, and notwithstanding the doubts which several very distinguished geognosts have founded upon these passages. %. classification of deposites marks great epochs of nature; for example, the appearance of some . animals (zoophytes, cephalopodous mollusca), and the simultaneous destruction of an enormous mass of monocotyledons. It presents as it were points of rest to the mind, and by keeping in view that the formations themselves are much less important than the great divisions, we have often an opportunity, on advancing from high mountains toward the plains, of o: the varied influence which the association of primitive and transition rocks, and that of secondary and tertiary ones, have exercised upon the inequality and configuration of the ground. It is owing to this influence, that the aspect of the landscape, the form of mountains and platforms, and the character of the vegetation, vary less, when we travel parallel to the direction of the beds, than on cutting them at a right angle. (Greenough, Crit. Exam. of Geology, p. 38).

f continue by following Messrs de Buch, Freiesleben, Brochant, Beudant, Buckland, Raumer, (Geb. von Nieder-Schles., 1819), and other celebrated . to group the independent formations according to the divisions, into primitive transition, secondary, and tertiary deposites, without troubling myself about the impropriety of the greater number of these denominations. I continue to separate the clay (with lignites) superimposed upon the chalk, from that which is beneath it, and the chalk itself from the more ancient secondary formations. But these distinctions, by beds and groups of beds, so useful in the description of a deposite of small extent, ought not to prevent the geognost, when he tries to rise to a more general point of view, from connecting these clays and the chalk with the Jura limestone, and from regarding them as the last strata of this great formation, composed of o: and marly beds. The inferior beds of the chalk (tuffeau) contain ammonities. The limestone of the mountain of St Peter of . Maestricht indicates, as has already been observed by Messrs Omalius and Brongniart (Geogr. Miner., p. 13), the passage of the chalk to older secondary limestones. Near Caen, according to the beautiful observations of M. Prevost, the clays beneath the chalk contain those same lignites which occur, in greater quantity, in the clay which is situated immediately above the chalk: cerites, which bring to mind the coarse limestone of Paris, are seen in a limestone with trigonias, placed between clays inferior to the chalk and the oolitic beds. I do not insist upon these particular facts; I mention them only to prove, by a striking example, how, on bringing together facts observed in different points of the same country, the great phenomenon of alternation reveals to us the connexions between formations which at first sight appear to have nothing in common. It is the property of those beds which alternate with one another, of those rocks which succeed each other in Periodical series, to present the most marked contrast in the two beds which immediately follow each other. In geognosy, as in the different parts of descriptive natural history, the advantage of classifications of variously graduated sections must be *cognized, without losing sight of the unity of nature; and those who have contributed the most to the advancement of natural philosoph , have possessed at once both the tendency * olize and the exact knowledge of a mass of particuas facts,

It has been customary to terminate the series of deposites by the volcanic rocks, and to make them succeed the secondary and tertiary, and even the alluvial deposites. In a system constructed upon the principle of relative antiquity, this arragement seems to me to have little to recommend it. It is without doubt the case that lithoid lavas are spread over the most recent formations, even over beds of gravel; nor can it be denied that there exist volcanic productions of different epochs: but, from what I have observed in the Cordilleras of Peru, of Quito, and of Mexico, in a part of the world so celebrated for the frequency of volcanoes, it seems to me, that the chief site of subterranean fires is in the transition rocks, and beneath those rocks. I have observed, that all the burning or extinct craters of the Andes open in the midst of trap porphyries or trachytes, (Berl. Abhandl. der Kön. Acad. 1813, p. 131), and that these trachytes are connected with the great transition porphyry, and syenite formation, According to this observation, it appears more natural to me to make the secondary and volcanic deposites to follow the transition deposite in a parallel manner, and as by bisection. By this new arrangement, the formation of porphyries, syenites, and greywackes, or that of transition porphyries, syenites, and granites, occurs connected at the same time; 1st, With the porphyries of the red sandstone in the secondary coal-deposite; 2dly, With the trachytes or trap porphyries which are destitute of quartz, and mixed with pyroxene. I employ with regret the term volcanic terrain, not that I doubt, like those who designate the trachytes, basalts snd phonolites (porphyrschiefer), by the name of trap terrain, that all which I have associated in the volcanic terrain has not been produced or altered by fire; but because several rocks, intercalated between the (primitive?) transition and secondary rocks, might also be volcanic. I would rather wish to avoid every (historical) idea of the origin of things, in a (statistical) view of relative situation or superposition. At Skeen, in Norway, a basaltic and porous syenite, containing pyroxenes, is placed, according to the observation of M. de §. between the transition limestone and the syenite with zircons. It is a bed, not a dike; and this is a much less problematical phenomenon than the basalt (Urgrünstein, Buch. Geogn. Beob. p. 124, and Raumer, Granit des Riesengebirges, p. 70), contained in the mica-schist of Krobsdorf in Silesia. The trachytes, with obsidian of Mexico, are intimately connected with the transition porphyries which alternate with syenites. The amygdaloid belonging to the red sandstone, assumes, on the Continent of Europe, and in

Equinoctial America, all the appearance of an amygdaloid of the basaltic formation. M. Boué, in his interesting Essai Geologique sur l'Ecosse, J. 126, 162, has described pyroxenic rocks (dolerites) included in the red sandstone. Without prejudging any thing regarding the origin of these masses, or in general regarding that of all the primitive and transition rocks, we designate here, by the name of Volcanic Terrains, the least interrupted series of rocks altered by fire. In drawing up the enumeration of the different rocks, I have made use of the names most generally employed by the geoosts of France, Germany, England, and Italy: in attempting to perfect the nomenclature of formations, I should be apprehensive of adding new difficulties to those which the discussion of relative positions already presents. I have, however, carefully avoided the denominations, too long preserved, of under and upper limestone; of gypsum of the first, second, or third formation, of old and new red sandstone, &c. These denominations without doubt present a true geognostical character; they have relation, not to the composition of rocks, but to their relative age. However, as the general type of the formations of Europe cannot be modelled aster that of a single district, the necessity of admitting parallel formations (sich terretende Gebirgsarten), renders the names of first or second gypsum, of old or middle sandstone, extremely vague and obscure. In one country it is proper to consider a bed of gy sum or of common sandstone as a particular formation; j. in another, it should be regarded as subordinate to neighboring formations. The geographical denominations are without doubt the best; they give rise to precise ideas of superposition. When it is .# that a formation is identical with the porphyry of Christiania, the lias of Dorsetshire, the sandstone of Nebra (bunter sandstein), the coarse limestone of Paris, these assertions leave no doubt in the mind of an experienced geognost, regarding the position which is to be assigned to the formation in question. It is also by silent convention, as it Were, that the words zechstein of Thuringia, Derbyshire Limestone, Paris Formations, &c. have been introduced into mineralogical language. They express a limestone which immediately succeeds the red sandstone of the coal deposite, a transition limestone placed beneath the coal sandstone, and lastly, fornations of more recent origin than the chalk. The o; difsculties which the multiplicity of these geographical denominations presents, consist in the choice of names, and in the "gree of certainty which may have been acquired, regarding the position or relative age of the rock to which the others are WOL. II.-No. 2. 15

« 이전계속 »