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
SILURIAN IN GEOLOGY.

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

latter. Following up this investigation in 1832, he established the great Merioneth anticlinal, 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 hills 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 Berwyns, 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 account 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. [1854] 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 underlaid by what he called the Longmynd and Gwastaden rocks. The non-fossiliferous strata of the Long

mynd hills in Shropshire were described as rising up to the east from beneath the Llandeilo rocks; and as appearing again in South Wales, at the same geological horizon, at Gwastaden in Breconshire, and to the west of Llandovery in Caermarthenshire; constituting an underlying series of contorted slaty rocks many thousand feet in thickness, and destitute of organic remains. The position of these rocks in South Wales was, however, to the north-west, while the strata of the Longmynd, as we have seen, appear to the east of the fossiliferous formations.

In the Philosophical Magazine for July, 1835, Murchison gave to the four formations above named the designation of Silurian, in allusion, as is well known, to the ancient British tribe of the Silures. It now became desirable to find a suitable name for the great inferior series, which, according to Murchison, rose from beneath his lowest Silurian formations to the northwest, and appeared to be widely spread in Wales. Knowing that Sedgwick had long been engaged in the study of these rocks, Murchison, as he tells us, urged him to give them a British geographical name. Sedgwick accordingly proposed for this great series of Welsh rocks, the appropriate designation of Cambrian, which was at once adopted by Murchison for the strata supposed by him to underlie his Silurian system. [Murchison, Anniv. Address, 1842; Proc. Geol. Soc. III., 641.] This was almost simultaneous with the giving of the name of Silurian, for in August, 1835, Sedgwick and Murchison made communications to the British Association at Dublin on Cambrian and Silurian Rocks. These, in the volume of Proceedings (pp. 59, 60) appear as a joint paper, though from the text they would seem to have separate. Sedgwick then described the Cambrian rocks of North Wales as including three divisions: 1. The Upper Cambrian which occupies the greater part of the chain of the Berwyns, where, according to him, it was connected with the Llandeilo formation of the Silurian. To the next lower division, Sedgwick gave the name of Middle Cambrian, making up all the higher mountains of Caernarvon and Merionethshire, and including the roofing-slates and flagstones of this region. This middle group, according to him, afforded a few organic remains, as at the top of Snowdon. The inferior division, designated as Lower Cambrian, included the crystalline rocks of the south-west coast of Caernarvon and a considerable portion of Anglesea, and consisted of chloritic and micaceous schists, with slaty quartzites and

been

subordinate beds of serpentine and granular limestone; the whole without organic remains.

These crystalline rocks were, however, soon afterwards excluded by him from the Cambrian series, for in 1838 [Proc. Geol. Soc. II, 679] Sedgwick describes further the section from the Menai Strait to the Berwyns, and assigns to the chloritic and micaceous schists of Anglesea and Caernarvon a position inferior to the Cambrian, which he divides into two parts; viz., Lower Cambrian, comprehending the old slate series, up to the Bala limestone beds; and Upper Cambrian, including the Bala beds and the strata above them in the Berwyn chain, to which he gave the name of the Bala group. The dividing line between the two portions was subsequently extended downwards by Sedgwick to the summit of the Arenig slates and porphyries. The lower division was afterwards subdivided by him into the Bangor group, (to which the name of Lower Cambrian was henceforth to be restricted, including the Llanberris roofing-slates and the Harlech grits or Barmouth sandstones; and the Festiniog group, which included the Lingula-flags and the succeeding Tremadoc slates.

In the communication of Murchison to the same Dublin meeting, in August, 1835, he repeated the description of the four formations to which he had just given the name of Silurian; which were, in descending order, Ludlow and Wenlock (Upper Silurian), and Caradoc and Llandeilo (Lower Silurian). The latter formation was then declared by Murchison to constitute the base of the Silurian system, and to offer in many places in South Wales distinct passages to the underlying slaty rocks, which were, according to him, the Upper Cambrian of Sedgwick.

Meanwhile, to go back to 1834, we find that after Murchison had, in his communication to the Geological Society, defined the relation of his Llandeilo formation to the underlying slaty series, but before the names of Silurian and Cambrian had been given to these respectively, Sedgwick and Murchison visited together the principal sections of these rocks from Caermarthenshire to Denbighshire. The greater part of this region was then unknown to Sedgwick, but had been already studied by Murchison, who interpreted the sections to his companion in conformity with the scheme already given; according to which the beds of the Llandeilo were underlaid by the slaty rocks which appear along their north-western border. When, however, they entered the region which had already been examined by Sedgwick, and reached the