GEOLOGY, CHART No. 2.
NOTES ON
SOME GENERAL PRINCIPLES IN GEOLOGY}
AND THEIR APPLICATIONS.
BY
CONTENTS.
A—EXPLANATIONS RELATING TO THE CONSTRUCTION OF M. BOUE’S GEOLOGICAL MAP OF THE GLOBE ,
B. —THE ELEVATION OF MOUNTAINS, AND THE CONTEMPORANEITY OF PARALLEL CHAINS,
C. -THEORETICAL CONSIDERATIONS RELATING TO THE ELEVATION OF MOUNTAINS,
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A.
EXPLANATIONS RELATING TO THE CONSTRUCTION OF M. BOUE’S GEOLOGICAL MAP OF THE GLOBE .
(CHIEFLY ABSTRACTED FROM M. BOUE’S MEMOIR IN THE “ BULLETIN DE LA SOCIETE GEOLOGIQUE DE FRANCE.” Tom. Premier, Deuxieme Seuie.)
Tiie work which—as revised afresh by M. Boue —is now presented to the student of the Physical Atlas, is one of those efforts, which only an Inquirer, combining extensive and accurate knowledge withsingular powers of generalizing, can be entitled to undertake: and as it is altogether novel, as well as peculiar in character, it will be necessary to append to it a longer explanatory statement than has beenrequired in our previous Numbers, so that its pretensions, authority, and import, may be rightly understood.
I.
Considering the greatness of his enterprise, and the imperfection of our knowledge of the details of the geological structure of the globe, Boue very wisely restricted himself to an attempt to trace the distri-bution and arrangement of six grand formations or groups of rocks; including, of course, in each formation, a long series of products and changes, connected by artificial, if not natural relations. The sixgroups, are these :—
i. the crystalline schistous formation, comprehending all the granitoid rocks.
II. the primary stratifications, or the transition series, including the carboniferous formation.
III. the secondary formation, extending erom the close of tiie carboniferous series to the close of the cretaceous.
IV. TnE TERTIARY formation.
V. THE ALLUVIUM, OR MODERN DETRITUS.
VI. VOLCANOES : IGNEOUS ROCKS OF THE TERTIARY AND ALLUVIAL EPOCHS ; AND SOME EXTRA-EUROPEAN PORPHYRIES AND DIORITES.
The colours indicating these different groups are explained on the map.
II.
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In the essay accompanying his work, as well as in the “ Guide du Geologue Voyageur,” Boue has given a list of his authorities in respect of the distribution of these six groups; and it is doing him onlyjustice, to say, that his list forms, at the present moment, our most complete bibliography of geological literature. But however ample these positive materials, they are yet far from sufficient to entitle any one toconstruct, by the mere arrangement of separate established facts, a geological map of the globe. Let every district, hitherto actually examined, be coloured on a map of the world,—and, assuredly, thesurfaces thus distinguished would appear very unimportant in magnitude compared with the others, exhibiting the absence of all knowledge derived from personal and approved inspection. Fortunately,however, the parts of the globe, whose constitution has been laid bare by actual scrutiny, manifest unequivocal marks of the prevalence of order and system in the arrangement of the various formations;and although the relations thus indicated are not yet in all respects perfectly defined, nor the irregularities or apparently exceptional cases connected with them sufficiently known, they still j'ustify us in drawinginferences from single facts, that are often highly probable. A map constructed, in a great measure, in reliance on laws and relations which are not absolutely established, cannot indeed pretend to minuteaccuracy: but instead of being therefore disregarded as the result of an ambitious fancy, or a mass of mere barren conj'ecture, it must—in so far as it conscientiously presents the sum of our positive knowledge,and rests its deductions only on the most general of the laws hitherto recognised—be received as one of those important and vigorous reconnaissances which are often the useful preludes and pioneers of soliddiscovery.
The facts from which the geological structure of an unexplored country may, with any degree of probability, be deduced, are manifestly those of Physical Geography . This branch of knowledge is merelya description of the forms into which great Revolutions have cast the present surface of the earth; and as these forms must be supposed to indicate the nature of their producing causes, similarity in the contourof distant portions of the earth’s surface should thus, in so far, indicate similarity in geognostical constitution. The expected analogy, indeed, often prevails even with regard to extremely minute features. Com-paring, for instance, the central spine of Asia, from the Taurus and Caucasus to the Pacific, with the Alpine masses of Europe , it is not difficult to perceive a most striking correspondence among all their depen-dencies. In each of the two continents lying between enormous hollows now filled by seas, we find a large central mass or nucleus, composed of schistous and granitic rocks, bifurcated at its extremities,where it exhibits transition and secondary formations, also found to the north. If Asia Minor, in so far corresponds with the Pyrenees and the ancient and partly igneous islet of central France, the Hun-garian and Styrian basins between the divisions of the Alps, are repeated in China on a very large scale; whilst the basin of the lower Danube corresponds with that of Tonquin, and other districts ofTurkey in Europe are virtually reproduced in the peninsula beyond the Ganges. On the shoulders of the Central Asiatic mass we find the same secondary chains as in the Alps ; and, in both continents, ter-tiary deposits fill up isolated basins among the mountains, in the form of low heights, between the secondary zone and the tertiary and alluvial plain. Regarded from the same point of view, the deserts of Turkes-tan and Mongolia are only a repetition of the great tertiary and alluvial valley to the north of the Alps; the immense valleys of the Ganges and lower Indus recall those of Piedmont and Venetian Lombardy;whilst the basins of the higher Euphrates, of Cabool, Cashmere, Ladak , Lassa, &c., have counterparts in the isolated tertiary cavities of Belluna, Carintbia, of the upper Muhr, the upper Inn, &c. Nor does the like-ness stop even here; for if the calcareous Alps of Asia enclose isolated tertiary basins like those of Europe , they exhibit also, in places precisely similar, clefts or passages opened by nature which have in bothinstances been the arteries of commerce, and the defiles whose possession has always ranked so important in war; and in localities exactly corresponding, we have the seats of civilization and power. It isscarce needful to refer to the enclosed basins of Persia, Tibet, Cashmere, &c.; the cols of the Hindoo Koosh olfer similitudes to the well-known cols of the Swiss Alps:—“ Delhi,” says Boue, “ is Milan , Calcuttareminds us of Venice, while Bombay is the counterpart of Genoa .”—Passing from the great central masses themselves to their environs, we are likewise everywhere met by close and unlooked-for analogies.North of the Asiatic protuberance beyond the cavity of the desert, the Altai presents a crystalline and transition region resembling what is found in Europe north of the tertiary depression borderingthe Alps; and in Asia , as in our own more neighbouring region, secondary and alluvial deposits divide the districts now referred to, from the older formations which constitute the northerly portion of bothcontinents. So also in the south, Spain seems the analogon of Arabia; Corsica, Sardinia and Italy, of Hindostan; and Greece and Turkey, of the peninsula beyond the Ganges . That these resemblances arenot fanciful, a glance at the map will readily manifest; nay, with due allowance for the effect of local and particular accidents, the analogy might be traced much farther—to the identification, for instance, of the
chain Soliman_the prolongation of the Bolor—with the Maritime Alps ; and even to the position of the immense hollows which, in both continents, are filled with water. Descending to still smaller minutiae,
14 is not easy to escape being struck by the singularly corresponding distribution of the more strictly igneous or volcanic rocks. Is not the great basaltic district of Hindostan , for instance, a manifest coun-terpart of the extensive eruptions of serpentine and trachyte in Italy and Turkey; while the volcanoes and their deposits in the Mediterranean and its Archipelago, and those of Hungary and Asia Minor , havesimilitudes in Cutch, the Sunda Isles, the Malayan Archipelago, Japan, and Kamtchatka ? North of the Alps we find only a few knolls or domes of igneous rocks of this family; now this accident seemsexactly repeated in Asia in the lakes of borate of soda ( e.g ., that of Tinkal in Tibet) : while, at a greater distance from the central chains, the ancient volcanic districts of the Eifel, the banks of the Rhine ,the north-west of Germany, and northern Bohemia, seem represented by elevated groups, such as the Caucasus , Ararat, and Demavend, the trachytes between the Caspian and Lake Aral, and the volcanic zonearound Lake Baikal .
This remarkable conformity of feature through two immense regions—a conformity as striking as the dissimilarity between the structure of either continent and the physical geography and geology ofAfrica and America —seems to establish, or at least to endow with a singular probability, what has been laid down as the fundamental idea of speculative geology, viz., that a critical similarity of outward form,while indicating similarity in the producing causes, must also, to a large extent, indicate identity of structure: but this conception by itself, being too general to further an enterprise so definite and determinateaS Ernies, he has deemed it necessary to enumerate specifically all those separate external features, from which specific and substantive inferences as to the geognosy of a district may with comparative securityhe made; and his sagacity and rare ingenuity appear to have enabled him to collect, methodize, and estimate the whole important considerations, which, in the present state of our knowledge, can yield theremotest hint regarding the structure of an unexplored region. The results arrived at are presented with slight modifications as to arrangement, but considerably abbreviated, in the next section.