mode of distributing minerals in a volcanic focus by the boiling process, may produce from the same material, rocks of a predominant feldspathic character in one place, and rocks of a hornblendic or augitic character in other places. Simple feldspathic granites may be fused and ejected as feldspathic rocks, like those of porphyry dikes. But it is an interesting fact, that the rock of most dikes is of the augitic (or hornblendic) kind, like the dikes of volcanoes that rise from sources in which the separating process could not have been operating.*

We also arrive at the important conclusion, that rocks perfectly compact in texture may be of subaërial origin, as we have pressure from the fluid lavas themselves in the volcanic focus.

Another deduction proceeds from the facts stated;—that the same igneous rocks may occur of all ages, provided the atmosphere or waters of the earth were not too warm for the more rapid rate of cooling required for uncrystalline rocks. Scorias, basalt, trap, porphyry, syenite, granite, have no relations to one epoch rather than another, beyond what may depend on the circumstance just mentioned. Whenever therefore in the history of the world, the variations in heat, pressure, and rate of cooling, now possible, may have taken place, similar rocks to those of the present day may have been in progress:-and as far as the variations of former times, in these respects, may now take place, former rocks and minerals may still be in progress. In this statement it is implied that the necessary elements are present in the fused material.

III. Origin of Continents.-The moon gives us hints on another topic of great interest, relating to the distribution of land and water on our globe. We have mentioned that there is a large area covering nearly one third of the hemisphere facing the earth, which is mostly free from volcanoes, while on other parts the craters are closely crowded together. We may therefore reason

* Mr. Darwin has accounted for the distribution of feldspathic and augitic rocks in volcanoes, on the ground of their different specific gravity. But with this cause alone, the lower parts of the feldspathic peaks should be expected to contain the heavier augitic material, which is not the case. He also argues that the feldspar would rise in the fluid as crystals, and so the augite sink. But we know in the first place, that crystals do not appear till incipient solidification, and if the augite and feldspar were both in distinct crystals, where would be the fusion? Again, the feldspar rocks are amorphous, except with a very slow rate of cooling; and how then can the existence of appreciable crystals be assumed?

ably infer, that over this naked portion, the surface first became solid, and has therefore cooled the longest and to the greatest depth. Consequently, the contraction from cooling, which was going on, would take place most rapidly over the thinner and more yielding volcanic portion; and unless the ejections made up the difference, this part would become somewhat depressed. A melted globe of lead or iron in the same manner, when cooling unequally, becomes depressed by contraction on the side which cools last. Now on our own globe, the continents have to a very great extent been long free from volcanic action. A glance at a map of Asia and America will make this apparent. It is usual to attribute this almost total absence of volcanoes from the interior of the continents to the absence of the sea; but it is fatal to this popular hypothesis, that the same freedom from volcanoes existed in the Silurian period, when these very continents were mostly under salt water, a fact to which the wide spread Silurian rocks of America and Russia testify. Over the oceans, on the contrary, all the islands excepting the coral, are igneous-and the coral may rest as we have reason to believe on an igneous base.

It is therefore a just conclusion that the areas of the surface constituting the continents were first free from eruptive fires. These portions cooled first, and consequently the contraction in progress affected most the other parts. The great depressions occupied by the oceans thus began; and for a long period afterward, continued deepening by slow, though it may have been unequal, progress. This may be deemed a mere hypothesis; if so, it is not as groundless as the common assumption that the oceans may have once been dry land, a view often the basis of geological reasoning.

Let us look farther at the facts. Before the depression of the oceanic part of our globe had made much progress, the depth would be too shallow to contain the seas, and consequently the whole land would be under water. Is it not a fact that in the early Silurian epoch nearly every part of the globe was beneath the ocean? So we are taught by the extent of the formations. The depth of water over the continental portions would be very various; but those parts which now abound in the relics of marine life, were probably comparatively shallow, as amount of pressure, light, and dissolved air, are the principal circumstances influencing the distribution of animals in depth, and acted formerSECOND SERIES, Vol. II, No. 6.—Nov., 1846.

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ly, we may believe, as at the present period. Here then we see reason for what has been considered a most improbable supposition, the existence of an immense area covered in most parts by shallow seas and so fitted for marine life.

If we follow the progress of the land, we find that with each great epoch there has been a retiring of the sea. In the coal deposits we have an abundant land vegetation. Subsequently, the progress on the whole was giving increased extent and height to the land and diminishing the area of the waters. Instead therefore of a bodily lifting of the continents to produce the apparent elevation, it may actually have been a retreating of the waters through the sinking of the ocean's bottom. The process however has not been a continuous one: for during each epoch,-the Silurian and the more recent,-there have been subsidences as well as seeming and actual elevations, and various oscillations of the continental surface, from subaërial to submarine and the reverse. When contraction had once taken place over the continents as well as under the ocean, there may have afterwards been expansions again through the return of heat from some cause. And thus various irregularities have taken place, such as the rocks indicate. In the tertiary period and since, the apparent rise of the land has been still to some extent in progress. And is there any evidence that this could have arisen from a sinking of the ocean's bed? The evidence is undoubted. For Mr. Darwin has shown satisfactorily, (and farther observations to the same end, and to many interesting conclusions, will be presented in the writer's geological report on the Pacific), that a subsidence of some thousands of feet has taken place since the corals commenced their growth. Every coral island is a register of this subsidence.

And why should not the ocean's bottom subside, as well as the land? What has given the continental portions of our globe their elevation, as compared with other parts, if not the unequal contraction of the whole? Can we safely affirm-in words of high authority" that the stability of the sea and the mobility of the land are demonstrated truths in geology," when mobile land forms also the bed of the ocean, and its changes must affect the

* See Silliman's Journal, xlv, 131, 1843.

Leonard Horner, Esq., Anniversary Address before the Geological Society of London, January, 1846; Quarterly Jour. of the Geol. Soc., No. 6, p. 199.

stability of the superincumbent waters; I ask, can we safely make this affirmation, until we know something more certain than past investigations have revealed, about the geological history of the two-thirds of the surface of our planet that are concealed beneath its oceans?

In our conclusion from the above reasoning, we fall in nearly with the views presented by a distinguished French geologist, M. C. Prevost, who has argued with much force in favor of subsidence as a cause of the apparent elevation of the land: though it may be right to state that these conclusions were arrived at previously to seeing his memoir.* There appear to be many objections to the opinions of M. Prevost, as they are expressed by him, inasmuch as no allowance is made or admitted for minor disturbances and actual elevations by subterranean forces. His views however are

well worthy the attention of the geological enquirer.

The principles explained place the general theory of change of level by contraction upon something better than a hypothetical basis, and are believed to explain the actual causes by which the changes have been produced. They correspond moreover with the view that ruptures, elevations, foldings and contortions of strata have been produced in the course of contraction. The greater subsidence of the oceanic parts would necessarily occasion that lateral pressure required for the rise and various foldings of the Alleganies and like regions.

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The general theory of changes of level by contraction and expansion, and the rise thus of continents, was first presented by Mr. Babbage and De la Beche. M. C. Prevost takes the different ground that all seeming elevations are the result of subsidence. His propositions are as follows, (Bulletin de la Soc. Geol. de France, xi, 1839 à 1840, p. 186) :

"1. Que le relief de la surface du sol est le résultat de grands affaissements successifs, qui, par contre-coup, et d'une manière secondaire, ont pu occasionner accidentellement des élévations absolues, des pressions latérales, des ploiements, des plissements, des ruptures, des tassements, des failles, etc.; mais que rien n'autorise à croire que ces divers accidents ont été produits par une cause agissant sous le sol, c'est-à-dire par une force soulevante;

"2. Que les dislocations du sol sont des effets complexes de retrait, de contraction, de plissement et de chute;"

"3. Que les matières ignées (granites, porphyres, trachytes, basaltes, lavas,) loin d'avoir soulevé et rompu le sol pour s'echapper, ont seulement profité des solutions de continuité qui leur ont été offertes par le retrait et les ruptures, pour sortir, suinter et s'épancher au-dehors."

ART. XXXI.-Description of three varieties of Meteoric Iron. -1. from near Carthage, Smith County, Tennessee; 2. from Jackson County, Tennessee; 3. from near Smithland, Livingston County, Kentucky; by G. TROOST, Prof. in the University of Nashville, Tennessee.

1. Meteoric Iron from Carthage, Smith County, Tennessee.

IN vol. xlix, p. 336, of this Journal, I published a description of four varieties of meteoric iron, one of which was of the highest interest as its fall had been witnessed by several persons. My collection has since been augmented by three other newly discovered specimens. A friend of mine, Samuel Morgan of Nashville, learned sometime in 1844 that a large mass of some metal had been found in Smith County near Carthage, Tennessee, which was considered as silver, and a small sample of it was given to him which we both recognized immediately as meteoric iron. Mr. Morgan immediately endeavored to learn its history and to get possession of it; but as I observed above, it being considered a precious metal, he failed, and every thing was enveloped in mystery, till it became known that it was not silver. He learned then that it was in the possession of a blacksmith, that it was found about a mile from Carthage the County seat of Smith County, and Mr. M. obtained it last year for a moderate price. It weighed 280 pounds-an oblong shapeless mass, its surface showing here and there some projecting octahedral crystals. A piece of it was sawed off weighing 39 lbs. which now forms one of the ornaments of my cabinet. This magnificent specimen has a polished surface of about 12 by 9 inches. None of the metallic meteorites, that I have seen, exhibit such beautiful Widmannstattean figures which have become visible on its polished surface, without the aid of acid. It shows rhomboidal and triangular sections which are generally a full inch, and a few, more than an inch, in length. These figures cover uniformly the whole of the polished surface. No heterogeneous materials are visible in it. There is only one cavity of about inch on its surface.

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The unpolished part is partly crystallized and partly amorphous and compact. Some crystals (parts of octahedrons) project for more than an inch above the mass. The iron is very tough and malleable, and, as it contains no traces of pyrites, not susceptible