of 64.34 p. c. The homogeneous, nearly infusible matrix of the VIII.-SPECIFIC GRAVITY. It has been already remarked that in general the specific gravity of original rocks decreases with the increase of silica and increases with the decrease in quantity of the same substance; the most acid rocks are specifically the lightest, the most basic rocks are specifically the heaviest. Abich was the first to call attention to this as exhibited among the volcanic rocks, and to shew the conclusions which might be drawn regarding the silica contents of these rocks from their ascertained specific gravities. Although the same relation has been observed to exist among the granitic and porphyritic rocks, and doubtless runs through all the orders, it has not been found that a certain specific gravity invariably corresponds to a certain degree of silicification or that, for instance, because a syenite containing 59.83 p. c. of silica has a specific gravity of 2,730, a trachyte having the same silica contents will have the same specific gravity. On the contrary we find decided differences as to specific gravity in rocks of similar composition, but belonging to different orders of texture. The following table shews the average specific gravity of the various families of granular, porphyritie and trachytic rocks: It will be observed from this table that the specific gravity of granular rocks is generally greater than that of the trachytic rocks which correspond with them in degree of acidity; granites are heavier than rhyolites, and greenstones than dolerites. (The rule does not hold good when applied to the basic rocks, but this may be owing to the facility with which they become decomposed and absorb water, which causes a material diminution of gravity.) The porphyritic rocks seem to occupy a position between the other two series, being neither so dense, relatively, as the granular nor so light as the trachytic rocks. This would seem to indicate that the coarsely gran lar rocks crystallised more slowly and perfectly than the porphyries and the latter more than the trachytes. This difference in density between rocks having the same percentage of silica is even more observable between trachytic and vitreous rocks. Obsidian has invariably a much less specific gravity than a quartzose trachyte which possesses the same percentage of silica. Thus we have the specific gravity of Rhyolite from Palmarola with 74.54 p. c. Si. O2 = 2·529 = 2.370 The cause of the difference seems merely to be that while the rhyolites cooled slowly and shrank together to a denser mass, the obsidians are quickly cooled unannealed natural glasses. It is well known that garnet, vesuvianite, orthoclase, labradorite, augite, and olivine have their densities much decreased by being fused and quickly cooled, and the same thing has been remarked with regard to rocks. St. Claire Deville, and Delesse experimented on several rocks, and found that their specific gravities were diminished after fusion. St. Claire Deville's results were as follows: ..... Basaltic lava from the Peak of los Majorquinés 2.945 2.836 2.879 2.662 2.360 Delesse found the loss to be less with fine-grained and semivitreous rocks than with those of a distinctly crystalline character. According to his results, if the rocks experimented on be arranged according to the degree of diminution which their specific gravities. undergo in fusion, beginning with those which experience greatest loss, those rocks will be found at the head of the list which are commonly considered to be the oldest in age. Delesse found the following per centages of diminution, the specific gravity of the various rocks before fusion being regarded as = 100. Granite, granulite and quartz porphyry....... 9-11 p. c. As early as 1841, Gustav Bischof made observations on the comparative volumes of Basalt, Trachyte and Granite in their crystalline, melted, and vitreous conditions, with the following Nothing can be more obvious from these data and experiments than that original rocks in cooling, solidifying and erystallising, underwent contraction, increasing thereby their density, and that the amount of contraction was the greater the more thoroughly and coarsely crystalline the rock, and the earlier the dates of its eruption in the geological history of the earth. It is not customary in treating of eruptive rocks usually to entertain any very definite ideas as to their age, but it ought not to be forgotten that the geological experience of Europe has shewn that they made their appearance on the earth's surface somewhat in the same order as they occupy in Table III. It would therefore seem that those rocks which have experienced most perfect crystallisation and the greatest amount of contraction or increase of density during that process are the oldest in geological age, that those which have crystallised imperfectly and experienced but a moderate amount of contraction, belong to the middle age of geological history, and that those which have solidified quickly to a semi-vitreous condition, and have experienced in so doing. scarcely any contraction, are exactly those which are the most. recent, and have been denominated volcanic rocks. Such results ought not to surprise us, but ought rather to be anticipated if the theory of the original igneous fluidity of the globe be well founded. The enormous degree of heat, which only could have occasioned such a condition, could not have disappeared suddenly. A gradual decrease of temperature must have taken place from the time when the solidification of the earth began down to recent geological periods. It follows that this gradually decreasing temperature must have had more or less influence upon the cooling of the various rocks protruded through the earth's crust during different geological ages. Those which appeared in earlier periods must have cooled when the earth's temperature was very high, and must therefore have enjoyed the most favorable conditions for slow and perfect crystallization and great contraction. of volume, while on the other hand, those which were erupted in later ages must have appeared at a time when the temperature had much diminished, and consequently they must have solidified much more rapidly, crystallised much more imperfectly, and experienced less increase of density than their predecessors. Thus there can be distinctly traced a very decided connection between the universally accepted theory of the earth's original fluid condition and many of the facts which have been here stated with regard to the density of original rocks. But although, generally, definite relations can be shewn to exist between the age and texture of rocks, it is not to be supposed that this is invariably the case, that there are no exceptions to the rule. It is not to be forgotten that other conditions besides the temperature of the earth's surface may have exerted their influence. Thus it is frequently the case that veins or dykes of diorite have in the centre a distinctly compound texture, while toward the sides they become almost impalpable. Then again beds of basaltite are often seen to be in the upper part and at the bottom fine-grained and compact, while in the middle they are small-grained and variolitic in texture. It is also frequently to be observed that masses of granite distinctly granular in the centre, assume towards the periphery a schistose texture, the direction of which is most generally parallel to the line of junction with the neighbouring rock. Thus it appears that in the solidification of a rock, the space which it occupied, the pressure to which it was exposed, the temperature of the enclosing rocks at the time of eruption, and the circumstances under which it was erupted, whether, for instance, on land or under water, must have influenced more or less its resulting density as well as its texture. HISTORY OF THE NAMES CAMBRIAN AND BY T. STERRY HUNT, LL.D., F.R.S. It is proposed in the following pages to give a concise account. of the progress of investigation of the lower paleozoic rocks during the last forty years. The subject may naturally be divided. into three parts: 1. The history of Silurian and Upper Cambrian in Great Britain from 1831 to 1854; 2. That of the still more ancient paleozoic rocks in Scandinavia, Bohemia, and Great Britain up to the present time, including the recognition by Barrande of the so-called primordial paleozoic fauna; 3. The history of the lower paleozoic rocks of North America. I. SILURIAN AND UPPER CAMBRIAN IN GREAT BRITAIN. Less than forty years since, the various uncrystalline sedimentary rocks beneath the coal-formation in Great Britain and in continental Europe were classed together under the common name of graywacke or grauwacké, a term adopted by geologists from German miners, and originally applied to sandstones and other coarse sedimentary deposits, but extended so as to include associated argillites and limestones. Some progress had been made in the study of this great Graywacke formation, as it was called, and organic remains had been described from various parts of it; but to two British geologists was reserved the honor of bringing order out of this hitherto confused group of strata, and establishing on stratigraphical and paleontological grounds a succession and a geological nomenclature. The work of these two investigators was begun independently and simultaneously in different parts of Great Britain. In 1831 and 1832, Sedgwick made a careful section of the rocks of North Wales from the Menai Strait across the range of Snowdon to the Berwyn hills, thus traversing in a south-eastern direction Caernarvon, Denbigh and Merionethshire. Already, he tells us, he had in 1831, made out the relations of the Bangor group, (including the Llanberris slates and the overlying Harlech grits,) and showed that the fossiliferous strata of Snowdon occupy a synclinal, and are stratigraphically several thousand feet above the horizon of the |