enormous that, as Hall justly remarks,* "the small number of restricted species reduces the importance of the system to the value of some of our subordinate groups." D'Orbigny, however, considers that the great number of species heretofore considered identical is due solely to incorrect determinations.

Where in England the line between Upper and Lower Silurian shall be drawn is another question which seems by no means satisfactorily settled, although Murchison remarks that "it was at the summit of this Caradoc sandstone that I long ago drew the line of demarkation between the inferior and superior masses of the Silurian systems and observations extended to many distant regions have confirmed the general truthfulness of that division." In the original classification of the Silurian by Murchison, the Caradoc and Llandeilo groups were bracketed together as Lower Silurian, and in the Siluria we are reminded that all the fossils which most frequently occur in the heart of the Caradoc formation are found in the Llandeilo, and yet the Government Surveyors have decided that the Caradoc must be taken from the Lower Silurian and placed as an intermediate group between that and the Upper division, at the same time allowing that the two cannot be palæontologically separated from each other. Sedgwick and M'Coy have shown that the rocks colored as Caradoc in the Malvern Hills by the Government Survey are filled with Wenlock fossils. Partly as a result of this confusion, we find figured in the Siluria, as characteristic of the Lower Silurian a number of fossils which in this country are universally recognised as Upper Silurian only; for instance, Pentamerus oblongus, Halysites catenulatus (Catenipora escharoides), Favosites Gothlandica and others.

The map attached to the Siluria, on which is represented the extent of the palæzoic rocks throughout the world, shows the remarkable fact that these, together with the Azoic or crystalline rocks, cover by far the larger part of the earth's surface. Judging by the eye it would appear that of the regions whose geology is approximately known, not more than one-fifth or one-sixth of the surface is occupied by the Mesozoic and Cainozoic, or Secondary and Tertiary, strata. It will not escape any one's notice that the predominating rocks in high northern latitudes are palæozoic; as far north as the most adventurous explorers have penetrated, Silurian and Carboniferous fossils have been found amid eternal snow and ice. The great uniformity in the character of organic life over so vast an extent of the globe during the palæozoic epoch, indicating, as it does, climatic conditions of a very different character from those which now prevail, is one of the most interesting of the revelations of geological science. The very small development of the older fossiliferous rocks in the equatorial zone

*Foster and Whitney's Report, Part II, p. 312.

is another important fact which seems to us to indicate that the conditions for the growth of organic life in that part of the earth were unfavorable during the earlier periods of animal and vegetable existence. If the internal heat of the earth be adopted, as is done by most geologists, as the principal cause of the more uniform and elevated temperature of the globe during the earlier geological periods, is it not a legitimate inference to conclude that the same causes which rendered the now frozen arctic zone sufficiently warm to support a prolific growth of plants and animals, must have so increased the temperature of the equatorial regions that life could not exist there except under peculiar and exceptional circumstances? Thus, the colder portions of the earth are by far the best provided with coal, and within the limits of the torrid zone there seems to be almost a total want of the proper coal-measures.

The order of the palæontological development of the paleozoic strata can nowhere be so well studied as in North America. The vast space over which the older fossiliferous rocks are spread out and the fact that, in spite of the immense period since the animals and plants entombed in them lived and flourished, they remain almost in the same position in which they were originally deposited, are two strong reasons why this study should be carried on in this country with vigor and success. The labors of James Hall have already shed a flood of light over this, the most interesting, department of our geology, and we look on the volumes of the "Palæontology of New York" as a not less important contribution to our knowledge of the palæozoic world than those of Murchison or Barrande. Much yet remains to be done in our great western valley, before our acquaintance with the history of the development of organic life in our older strata will have reached anything like a satisfactory stage. When the various groups, which have been so thoroughly studied in New York by Hall, shall have been equally well worked out in their western and southwestern extension, we shall have a more complete picture of palæozoic life, as developed under every variety of physical condition, than any part of Europe can furnish. Thanks to the labors of Mr. Logan in Canada, we are fast gaining a knowledge of that interesting region thus getting a clue to some of the intricacies of New England geology, so that we may hope that so large a part of our northeastern states will not always remain but little better than a blank upon our geological maps.

The subject of the original formation and distribution of gold over the earth's surface is one in which Murchison has, since his visit to the Ural, been particularly interested, and a chapter of the Siluria is devoted to a recapitulation of what he has elsewhere published in regard to it. In 1844 our author was led, principally by an examination of specimens collected by Count

Strzelecki along the eastern mountain range of Australia to infer that there was a strong resemblance between that region and the Russian auriferous chain, and he adverted to this in his address before the Geographical Society in that year, suggesting that gold would probably be found there. It had indeed been already discovered by Strzelecki, as early as 1839; but at the earnest request of the Governor of New South Wales this gentleman has kept the matter a profound secret.* In 1846 Murchison received specimens of auriferous quartz from that region, and he again enlarged on the analogy between the Australian Cordillera, as he termed it, and the Ural chain, urging the Cornish miners to go out and seek their fortune as gold washers in that distant country. Since that time, he has written numerous papers, the chief object of which has been to set forth the facts of his connection with the gold discoveries in Australia and to show that the public need be under no apprehension of a fall in the value of the precious metal, on account of its immensely increased production since the opening of the auriferous regions of California and Australia. This increase, according to Murchison, can only continue for a short time, since the records of mining have shown that auriferous veins rapidly decrease in richness from the surface downwards, so that they cannot be worked with profit, while the detrital beds, from which almost all the gold is obtained, are necessarily limited in depth and are therefore soon exhausted, especially when Anglo-Saxon energy is directed to their exploration. Our author's predictions have been more completely sustained in the case of Australia than in that of California. The yield of gold from the former country is already falling off rapidly, but the Californian gold-fields, although they were attacked four years before those of Australia are still yielding only a slightly diminished quantity. Still there is no doubt that the maximum of production has been reached, and that there will be a decline, probably slow and gradual, but still inevitable. It seems hardly possible to avoid the conclusion that enough gold will have been obtained in the course of the twenty years following the opening of California, to produce a sensible effect in depressing the value of the precious metal. The auriferous sedimentary and detrital beds are accumulated on a more gigantic scale along the flanks of the Sierra Nevada than anywhere else, and a very considerable time must elapse before they will have become exhausted of their metalliferous contents, while it is by no means proved

* It is worthy of note that the authorities of New South Wales again and again endeavored to suppress the knowledge of the existence of gold in that region, lest it should interfere with sheep-growing. Although Strzelecki was actually the first to find gold; yet, practically, the discovery is due to Mr. Hargraves, a "returned Californian," who with a little of the "go-ahead" spirit brought from the other side of the world commenced "prospecting" and digging, without much regard for governors or sheep.

that a very considerable amount of the precious metal may not be obtained from workings in the veins themselves, if properly and economically managed.

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Within the last few years, the ratio of the production of gold to silver has undergone a great change. Early in the present century it was by weight nearly as I of gold to 43 of silver; in 1845 when the Siberian washings were most productive, the ratio was 1:17; in 1852, when California and Australia were yielding most largely, it was as 1:4. It has been generally admitted by writers on political economy, that so great a change as this in the relative amount of the two metals brought into must have an effect on their relative values, and that silver must gradually rise to something approaching the value which it formerly had as compared with gold. Murchison however has the extraordinary idea that "Providence seems to have originally adjusted the relative value of these two precious metals and that their relations having remained the same for ages, will long survive all theories." He seems to forget, that the opening of the silver mines to South America in the sixteenth century so increased the produce of that metal as to depress its value, as compared with gold, from a ratio of about one to ten, which it had maintained for a long period, to one to fifteen. Since the earliest historical times the relative value of the two noble metals has always been more or less fluctuating, and, being dependent on the varying influences of demand and supply, it will no doubt always continue so; although, except under extraordinary circumstances, such as the discovery of the California gold-fields may be conceived to be, usually vibrating within narrow limits.

The last chapter of the Siluria is chiefly devoted to a general view of the succession of life from a beginning, as based on positive observation, carefully distinguishing absolute geological results from mere theoretical speculations on what may hereafter be found. The idea of a progressive development in animal and vegetable life, which was for a time so stoutly battled against, seems to be more generally recognised. It is now seen that in the general anxiety which once prevailed among geologists to bring down the higher types of organized existence into the lowest strata, a good many mistakes were made, which more careful observations have set right. Thus the famous tracks in the Potsdam sandstone of Canada, which were once so confidently pronounced to belong to Chelonians, are now regarded as Crustacean. At one time we were led to believe that fishes made their appearance as low down in the series as the Lower Silurian, and this was strongly insisted on as opposed to the theory of progression. It now seems, however, to be almost certain from the comparison of observations made all over the world, that it is not until we rise to the uppermost beds of the Silurian System that we find

traces of vertebrate life, in the form of a few minute fishes. In England the earliest fishes are now supposed to occur in the upper Ludlow, the so-called "fish defences" in the lower rocks, having been shown to be crustacean. The fragments of jaws, teeth and skin found in the "bone-bed" of the upper Ludlow, however, do not seem to be capable of being referred with absolute certainty to the class of fishes, since the opinions of palæontologists are divided with regard to the larger part, if not all, of them.

Our author, therefore, seems to us justified in asserting that, looking at the Silurian System as a whole, we know that its chief deposits (certainly all the lower and most anterior) were formed during a long period, in which, while the sea abounded with countless invertebrate animals, no marine vertebrata had been called into existence; that these may yet be found is certainly not impossible; but every year of active exploration all over the world diminishes the probability of such an event, and should make us more unwilling to accept as a fact any supposed discovery of the kind, unless substantiated by undoubted evidence.

In taking leave of our author, we feel strongly impressed with the change which under his guidance, has been wrought in our knowledge of the lower fossiliferous strata during the last few years; the impulse which his labors have given to palæozoic geology is everywhere felt and acknowledged. Still we close the pages of the "Siluria" feeling quite as deeply that if much has been done, much more remains to do; the outlines have been drawn upon the canvass, but the perfect picture will only be the result of long-continued and associated labor. The geologists of this country especially have a noble task before them; their field is almost unlimited, the results must be proportionally grand.-J. D. w.

ART. XXXVIII.-Barometric Anomalies about the Andes; by Lieut. M. F. MAURY, U. S. N.*

LIEUT. HERNDON, U. S. N., in his descent of the Andes, in 1851-52, on his way from Lima to explore the valley of the Amazon and to descend that river to the Atlantic, determined the heights of various places above the level of the sea, both by barometric pressure, and by the boiling point of water. His boiling apparatus was constructed by Mr. Wm. Wurdemann, of Washington. His observations as to atmospheric pressure, made with the view of determining heights above the sea level, appear to indicate * From Maury's Sailing Directions, 4to, 1853.

SECOND SERIES, Vol. XIX, No. 57.-May, 1855.

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