Изображения страниц

It will be observed from this table that the specific grarity 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 granular 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
Obsidian from Lipari with 74.05

= 2.370
Quartz trachyte from Besobdal,
Asia Minor, with 76.56

= 2.656
Obsidian from Little Ararat with 77.27

= 2.394

[ocr errors]

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 garpet, 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:

Specific Specific


before fusion. after fusion. Vitreous lava from the Peak of Teneriffe.. 2.570 2.464 Trachyte from Chahorra ...

2.727 2.617 Basaltic lava from the Peak of los Majorquines 2.945 2.836 Basalt from Pic de Foga, Cape of Good Hope.. 2.971 2.879 Granite from Andoux..

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




[ocr errors]


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.
Syenitic granite, and syenite
Porphyry with orthoclase and oligoclase, with
and without quartz...

Diorite and diorite porphyry.
Basalt, trachyte, and old volcanic rocks

3Lavas and volcanic rocks.... As early as 1811, Gustav Bischof made observations on the comparative volumes of Basalt, Trachyte and Granite in their crystalline, melted, and vitreous conditions, with the following results :

Volume in vitreous condition. in crystalline.
Basalt ....



5 0-4




Volume in a fluid state.

1 1 1

in crystalline.

0.8960 0.8187 0.7431

Nothing can be more obvious from these data and experiments than that original rocks in cooling, solidifying and crystallising, 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 entert:in 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 ex:ictly 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,



By T. STERNY 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.


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 latter. Following up this investigation in 1832, he established the great Merioneth anticlipal, which brings up the lower rocks on the south-east side of Snowdon, and is the key to the structure of North Wales. From these, as a base, he constructed a section along the line already indicated, over Great Arenig to the Bala limestone, the whole forming an ascending series of enormous thickness. This limestone in the Berwyn bills is overlaid by many thousand feet of strata as we proceed eastward along the line of section, until at length the eastern dip of the strata is exchanged for a westward one, thus giving to the Berwyn chain, like that of Snowdon, a synclinal structure. As a consequence of this, the limestone of Bala re-appears on the eastern side of the Berwyus, underlaid as before by a descending series of slates and porphyries. These results, with sections, were brought before the British Association for the Advancement of Science at its meeting at Oxford, in 1832, but only a brief and imperfect ac. count of the communication of Sedgwick on this occasion appears in the Proceedings of the Association. He did not at this time give any distinctive name to the series of rocks in question. [L E. & D. Philos. Mag. [1851] IV, viii, 495.]

Meanwhile, in the same year, 1831, Murchison began the examination of the rocks on the river Wye, along the southern border of Radnorshire. In the next four years he extended his researches through this and the adjoining counties of Hereford and Salop, distinguishing in this region four separate geological formations, each characterized by peculiar fossils. These formations were moreover traced by him to the south-westward across the counties of Brecon and Caermarthen ; thus forming a belt of fossiliferous rocks stretching from near Shrewsbury to the mouth of the river Towey, a distance of about 100 miles along the north-west border of the great Old Red sandstone formation, as it was then called, of the west of England.

The results of his labors among the rocks of this region for the first three years were set forth by Murchison in two papers presented by him to the Geological Society of London in January, 1834. [Proc. Geol. Soc. II., 11.] The formations were then named as follows in descending order: 1. Ludlow, 2. Wenlock, constituting together an upper group; 3. Caradoc, 4. Llandeilo (or Builth) forming a lower group. The Llandeilo formation, according to him, was underluid by what he called the Longmynd and Gwastaden rocks. The non-fossiliferous strata of the Long

« ПредыдущаяПродолжить »