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These crystals, which were prepared by Mr. R. Phillips, and are very minute, have one distinct cleavage parallel to the plane P, and apparently another parallel to M.

The primary form is a doubly oblique prism, and the measurements are as follows:

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The primary form is an oblique rhombic prism. There are distinct cleavages parallel to the planes M and M', but I have not observed any other. The crystals are usually attached by one of the lateral ends of the figure, in consequence of which the planes P, a, and c, appear like lateral planes of a prism, and M, M", as its dihedral termination.

Fig. 1 exhibits the common form of the crystals; and fig. 2 a modified form which sometimes occurs, and not unfrequently with only one of the planes e apparent at the lateral extremity, the other not being visible.

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Cleaves readily parallel to the planes M, M', and h, of the annexed figure, but I can observe no cleavage in any other direction.

From the character of the secondary planes, the primary form is a right rhombic prism, and the measurements taken chiefly on a crystal I received from M. Teschemacher, are nearly those

which follow. The crystals, however, so speedily lose their brilliant surfaces when exposed to the air, or even when inclosed in a bottle, that the measured angles of the secondary faces are less to be relied upon than those afforded by the cleavage planes.

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The crystalline form assigned by the Abbé Hauy to sulphate of iron is a rhomboid; but it was, I believe, first observed by Dr. Wollaston, that its true form was an oblique rhombic prism. I do not find any published account of the ordinary figure of the crystals, or of the measurements of the planes; and as its form approaches very nearly to that of sulphate of cobalt, I am induced to give the measurements of both substances in reference to the annexed figure.

In sulphate of cobalt another plane sometimes appears as e2, which measures about 124° with P. And in both these sulphates

ez

·M

P

ez

a

there are also other planes a and e, which occur on some of the

crystals.

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The primary form has been determined from some very perfect and brilliant crystals which I have received from M. Teschemacher, and the measurements given below have very nearly coincided on several of these.

There is a distinct cleavage parallel to the plane h, but apparently in no other direction. The primary form inferred from that of the crystals, as shown in fig. 1, is a right rhombic prism.

For this salt I am indebted to Mr. Cooper.

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Fig. 2 represents one of the varieties of intersected crystals which occur very frequently among the single ones, the nature of which will be readily understood from the similar letters placed on the corresponding planes.

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On the Constitution and Mode of Action of Volcanoes, in different Parts of the Earth. By Alexander Von Humboldt.*

WHEN We consider the influence which scientific travels into distant regions, and a more extended geographical knowledge, have for some centuries past exerted upon the study of nature, we soon discover how this influence has varied according to the objects of inquiry, which have been, on the one hand, the forms of the organic world, and, on the other, the inanimate formation of the earth ;the knowledge of rocks, their relative ages, and origin. Different forms of plants and animals enliven the earth in every zone, as well in the plains, where the heat of the atmosphere is determined by the geographical latitude and the different inflexions of the isothermal lines, as where it changes suddenly on the steep declivities of the mountains. Organic nature gives a peculiar physiognomical character to every zone, which is not the case with the inorganic world where the solid crust of the earth is divested of its vegetable covering. The same rocks approaching

* Read before the Royal Academy of Sciences of Berlin, Jan. 24, 1823,

to and receding from each other in groups occur in both hemispheres, from the equator to the poles. On a distant island, surrounded by strange plants, under a sky where the wellknown stars do not shine, the sailor recognises, often with glad surprise, the clayslate which is the common rock of his native country.

This independence of the geognostical relations of places on the present constitution of their climate, does not diminish, but only gives a particular direction to the favourable effect upon the progress of geology and physical geognosy, which is produced by numerous observations made in foreign countries. Every expedition enriches natural history with new plants, and new genera of animals; at one time they are organic forms ranging themselves with well-known types, and representing to us, in its original perfection, a regularly woven, though often apparently interrupted texture of animated creatures; at another, they are forms which appear to be isolated, as vestiges of genera which have been destroyed, or as surprising members of groups still to be discovered. Such a variety is not presented by the examination of the solid crust of the earth; it rather reveals to us an agreement, which excites the admiration of the geognost, between the parts of which it is composed, in the superposition of masses of different natures, and in their periodical repetition.

In the chain of the Andes, as well as in the central mountains of Europe, one formation seems, as it were, to occasion the existence of another; masses of the same character assume similar forms: mountains are formed by basalt and dolerite; steep declivities by dolomite, porphyry, and quadersandstein; bell-shaped eminences and high-vaulted domes by vitreous trachyte rich in felspar.

In the most distant zones, larger crystals, as it were by internal evolution out of the more compact texture of the greater mass, aggregate into subordinate beds, and thus frequently announce the vicinity of a new and independent formation. Thus is the whole inorganic world reflected, more or less clearly, in every mountain of considerable extent; but in order to ascertain completely the most important phænomena respecting the composition, the relative age, and the origin of the different species of rocks, observations from the most distant parts of the earth must be compared together. Problems which had appeared enigmatical to the geognost in his mother country are solved near the equator. If distant zones do not furnish new species of rocks, that is to say, unknown arrangements of simple substances, as has already been remarked, they yet teach us how to discover the great laws which are every where the same, and according to which, the different strata of the earth support each

* In an imperfect translation of this paper, which has been forwarded to the Editor from the Continent, a word here occurs which cannot be decyphered; and on account of other inaccuracies which it has been necessary to correct, unaided by the original, the translation, as now given, is not to be regarded as exact in every particular.

other, appear in the form of veins, or are elevated by elastic

powers.

We need not be surprised, that, notwithstanding the great assistance which our geological information derives from inquiries, having whole countries for their object, an extensive class of phænomena (with which I venture to entertain this assembly), has been treated, during so long a period, in a confined manner; the points of comparison being more difficult, and, I might say, more troublesome to find. Whatever we believed we knew, until the end of the last century, respecting the form of volcanoes, and the action of their subterraneous forces, had been derived from two mountains of the south of Italy,-from Etna, and from Vesuvius. The first being more accessible, and having, like all low volcanoes, more frequent eruptions, has served for a type, according to which a whole distant world,-the powerful volcanoes of Mexico, South America, and the Asiatic Islands, has been considered. Such a method recalls to our remembrance the shepherd of Virgil, who expected his narrow cottage to contain the ideal of the eternal city, imperial Rome.

A careful examination of the whole Mediterranean, and principally of its easterly islands and shores, where mankind first awakened to mental culture, and to noble feelings, might certainly have dispelled such a narrow idea of nature. Out of the deep bed of the sea, among the Sporades, rocks of trachyte have arisen, like the Azoric island, which has thrice reappeared during three centuries, the intervening periods being almost equal. Between Epidaurus and Troezene, near Methone, the Peloponnesus has a Monte Nuovo which has been described by Strabo, and seen by Dodwell, higher than the Monte Nuovo of the Campi Phlegræi, near Baia; perhaps higher than the new volcano of Xorullo in the plains of Mexico, which I have found among a thousand basaltic cones, raised out of the earth, and still smoking. In the bason of the Mediterranean Sea also, the volcanic fire bursts forth, and not only from permanent craters, from isolated mountains which preserve a lasting communication with the interior of the earth, like Stromboli, Vesuvius, and Etna; -on Ischia, near the Epomæus, and also, as it would appear from the reports of the ancients, near Chalcis in the Lelantic plains, has lava flowed out of fissures which have suddenly opened. Besides these phænomena, which have taken place in the period of history within the narrow limits of certain traditions, and which Ritter will collect and explain in his masterly Geography, the shores of the Mediterranean contain abundant remains of more ancient igneous effects. The south of France shows, in Auvergne, a range of hills, in which bells of trachyte occur alternately with cones of eruption, from which currents of lava have descended. The Lombardic plain, which forms the innermost bay of the Adriatic Sea, surrounds the trachyte of the Euganean Hills, where domes of granular trachyte, of obsidian, and of pearlstone, rise, which, passing into each other, break through the Jura

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