moment can be regarded as fixing the date of the first introduction of any one class of beings upon the earth. Dates of the Discovery of different Classes of Fossil Vertebrata; showing the gradual Progress made in tracing them to Rocks of higher antiquity. in the gypsum some years before; but they were determined osteologically, and their true geological position was assigned to them in this memoir. 2 In 1818, Cuvier, visiting the Museum of Oxford, decided on the mammalian character of a jaw from Stonesfield. See also above, p. 408. 9 Plieninger, Prof. See above, p. 432. M. Darcet discovered, and Lamanon figured, as a fossil bird, some remains from Montmartre, afterwards recognized as such by Cuvier (Ossemens Foss., Art. "Oiseaux"). Owen, Prof., Geol. Trans., Second Ser., vol. vi. p. 203, 1839. The fossil bird discovered in the same year in the slates of Glaris in the Alps, and at first referred to the chalk, is now supposed to belong to the Nummulitic beds, and may therefore be of newer date than the Sheppey Clay. A bird's bone is also recorded by Mr. Prestwich as having been found by M. de la Condamine in the Upper part of the Woolwich beds. (Quart. Geol. Journ., vol. x. p. 157.) Early in 1855 the tibia and femur of a large bird equalling at least the ostrich in size were found at Meudon near Paris, at the base of the Plastic Clay. This bird, to which the name of Gastornis Parisiensis has been assigned, appears, from the Memoirs of MM. Hébert, Lartet, and Owen, to belong to an extinct genus. Professor Owen refers it to the class of wading land-birds rather than to an aquatic species. (Quart. Geol. Journ., vol. xii. p. 204, 1856.) Mr. Louis Barrett found many parts of the skeleton of a bird of the gull tribe in the coprolitic bed, in the Upper Greensand (see above, p. 332). The Archeopteryx macrura, Owen, was determined to be a bird by Owen in 1863. It occurred in the lithographic stone of Solenhofen, in which a single feather, probably of the same bird, had previously been found (see above, p. 396). 10 The fossil monitor of Thuringia (Protorosaurus Speneri, V. Meyer) was fig. ured by Spener, of Berlin, in 1810. (Miscel. Berlin.) 11 See above, p. 506. 12 Memorabilia Saxoniæ Subterr., Leipsic, 1709. 13 History of Rutherglen, by Rev. David Urc, 1793. 14 Sedgwick and Murchison, Geol. Trans., Second Ser., vol. iii. p. 141, 1828. 15 Sir R. Murchison. See above, p. 551. 10 Mr. Lee, of the Priory, Caerleon (see above, p. 555), found Pteraspis in presence of Mr. Lightbody, F.G.S. Obs.-The evidence derived from footprints, though often to be relied on, is omitted in the above table, as being less exact than that founded on bones and teeth. There are many writers still living who, before the year 1854, generalized fearlessly on the non-existence of reptiles in times antecedent to the Permian; yet in the course of nineteen years they have lived to see the remains of reptiles of more than one family exhumed from various parts of the Carboniferous series. Before the year 1818, it was the popular belief that the Palæotherium of the Paris gypsum and its associates were the first warm-blooded quadrupeds that ever trod the surface of this planet. So fixed was this idea in the minds of the majority of naturalists, that, when at length the Stonesfield Mammalia were brought to light, they were most unwilling to renounce their creed. First, the antiquity of the rock was called in question; and then the mammalian character of the relics. But when at length all controversy was set at rest on this point, the real import of the new revelation, as bearing on the doctrine of progressive development, was far from being duly appreciated. Their significance arose from the aid they afforded us in estimating the true value of negative evidence, when adduced to establish the non-existence of certain classes of animals at given periods of the past. Every zoologist will admit that between the first creation and the final extinction of any one of the oolitic mammalia now known, whether at Stonesfield or Purbeck, there were many successive generations; and, even if the geographical range of each species was very limited (which we have no right to assume), still there must have been several hundred individuals in each generation, and probably when the species reached its maximum, several thousands. When, therefore, we encounter for the first time in 1854 two or three jaws of Stereognathus or Spalacotherium, after countless specimens of Mollusca and Crustaceæ, and many insects, fish, and reptiles had been previously collected from the same beds, we are not simply taught that these individual quadrupeds flourished at the eras in question, but that thousands, perhaps hundreds of thousands, of the same species peopled the land without leaving behind them any trace of their existence, whether in the shape of fossil bones or footprints; or, if they left any traces, these have eluded a long and most laborious search. Moreover, we must never forget how many of the dates given in the above table (p. 588) are due to British skill and energy, Great Britain being still the only country in the world in which mammalia have been found in oolitic rocks. And if geology had been cultivated with less zeal in our island, we should know very little as yet of two extensive assemblages of tertiary mammalia of higher antiquity than the fauna of the Paris gypsum (already cited as having once laid claim to be the earliest that ever flourished on the earth)— namely, first, that of the Headon series (see above, p. 284), and, secondly, one long prior to it in date, and antecedent to the London Clay. This last has already afforded us indications of Cheiroptera, Pachydermata, and Marsupialia (see p. 292). How then can we doubt, if the globe were to be studied with the same diligence, if the six great continents, Europe, Asia, Africa, North and South America, and Australia, were equally well known, that every date assigned by us in the above Table for the earliest recorded appearance of fish, reptiles, birds, and mammals, would have to be altered and shifted back? Nay, if one other area, such as part of Spain, of the size of England and Scotland, were subjected to the same scrutiny (and we are still very imperfectly acquainted even with Great Britain), each class of vertebrata would perhaps recede one or more steps farther back into the abyss of time; fish might penetrate into the Lower Silurian-reptiles into the Upper Devonian-mammalia into the Lower Trias-birds into the Middle Oolite-and, if we turn to the Invertebrata, Trilobites and Cephalopods might descend into the Lower Cambrian-and Foraminifera into rocks now styled Azoic, and older than the Lower Laurentian. Yet, after these and many more analogous revisions of the Table, the order of chronological succession in the different classes of fossil animals would probably continue the same as now;—in other words, our success in tracing back the remains of cach class to remote eras would be the greatest in fishes, next in reptiles, and least in mammalia and birds. We have of late years acquired striking proofs of the difficulty of detecting the bones of man in those strata in which the works of his hands in the shape of flint implements abound. There are also large tracts of Eocene rocks very prolific of shells and other organ isms, as in Belgium, for example, which have been diligently studied for nearly a century without yielding a single bone of a mammifer. In the whole world the cretaceous and oolitic rocks have each of them only afforded as yet a single example of a fossil bird. It would almost seem as if the higher the type of organization the more powerful the spell required to evoke the remains of a fossil being from its stony sepulchre. "Unwilling I my lips unclose Leave, oh! leave me to repose." That we should meet with ichthyolites more universally at each era, and at greater depths in the series, than any other class of fossil vertebrata, would follow partly from our having as palæontologists to do chiefly with strata of marine origin, and partly because bones of fish, however partial and capricious their distribution on the bed of the sea, are nevertheless more easily met with than those of reptiles or mammalia. In like manner the extreme rarity of birds in Recent and Pliocene strata, even in those of freshwater origin, might lead us to anticipate that their remains would be obtained with the greatest difficulty in the older rocks, as the Table proves to be the case—even in tertiary strata, wherein we can more readily find deposits formed in lakes and estuaries. The only incongruity between the geological results and those which our dredging experiences might have led us to anticipate à priori, consists in the frequency of fossil reptiles, and the comparative scarcity of mammalia. It would appear that during all the secondary periods, not even excepting the newest part of the cretaceous, there was a greater development of reptile life than is now witnessed in any part of the globe. The preponderance of this class over the mammalia may have depended in part on climatal conditions, but it seems also clearly to imply the limited development, if not the total absence, before the Tertiary period, of the placental mammalia, whether terrestrial or aquatic, which, when they became dominant, acquired power to check and keep down the class of vertebrata nearly allied to them in structure, and coming most directly in competition with them in the struggle for life. For notwithstanding the impossibility of assigning even conjectural limits to the chronological extension of each class of vertebrata as we trace them farther and farther back into the past, it cannot be denied that our failure to detect signs of them in older strata, in proportion to the rank of their organization, favors the doctrine of development, or at least of the successive appearance on the earth of beings more and more highly organized, culminating at last in the advent of Man himself. CHAPTER XXVIII. VOLCANIC ROCKS. Trap Rocks-Name, whence derived-Their igneous origin at first doubted-Their general appearance and character-Volcanic cones and craters, how formed— Mineral composition and texture of volcanic rocks-Varieties of felsparHornblende and augite-Isomorphism-Rocks, how to be studied-Basalt, trachyte, greenstone, porphyry, scoria, amygdaloid, lava, tuff-AgglomerateLaterite-Alphabetical list, and explanation of names and synonyms, of volcanic rocks—Table of the analyses of minerals most abundant in the volcanic and hypogene rocks. THE aqueous or fossiliferous rocks having now been described, we have next to examine those which may be called volcanic, in the most extended sense of that term. Suppose a a, in the annexed diagram, to represent the crystalline formations, such as the granitic and metamorphic; bb the fossiliferous strata; and ece the volcanic rocks. These last are sometimes found, as was explained in the first chapter, breaking through a and b, sometimes overlying both, and occasionally alternating with the strata bb. They also are seen, in some instances, to pass insensibly into the unstratified division of a, or the Plutonic rocks. When geologists first began to examine attentively the structure of the northern and western parts of Europe, they were almost entirely ignorant of the phenomena of existing volcanoes. They found certain rocks, for the most part, without stratification, and of a peculiar mineral composition, to which they gave different names, such as basalt, greenstone, porphyry, and amygdaloid. All these, which were recognized as belonging to one family, were called "trap" by Bergmann, from trappa, Swedish for a flight of steps-a name since adopted very generally into the nomenclature of the science; for it was observed that many rocks of this class occurred in great tabular masses of unequal extent, so as to form a succession of terraces or steps on the sides of hills. This configuration appears to be derived from two causes. First, the abrupt original terminations of sheets of melted matter, which have spread, whether on the land or bottom of the sea, over a level surface. For we know, in the case of lava flow |