Maj 2, 1872] 



NATURE 



15 



HISTORY OF THE NAMES CAMBRIAN AND 



SILURIAN IN GEOLOGY* 



T T is proposed in ihe following; pages to give a concise account 

 ■^ of the progress of investigation of the lower palajozoic rocks 

 during the last forty years. The subject may naturally be divided 

 into three parts : (i) the history of Silurian and Upper Cambrian 

 in Great Britain from 1S31 to 1854; (2) that of the still more 

 ancient palaeozoic rocks in Scandinavia, Bohemia, and Great 

 Britain up to the present time, including the recognition by Bar- 

 randeof the so-called primordial palreozoic fauna; (3) the his- 

 tory of the lower palaeozoic rocks of North America. 



I. — SiliiriiDt and Upper Cambrian in Great Britain. 

 Less than forty years since, the various uncrystalline sedimen- 

 tary rocks beneath the coal- formation in Greit Britain and in 

 continental Europe were cla!;sed together under the common 

 name of graywacke or grauwacke, a term adopted by geologists 

 from German miners, and originally applied to s.uidstones and 

 other coarse sedimentary deposits, but e.xtended so as to include 

 associated argillites and limestone;. Some progress had been 

 made in the siudy of this great Graywacke formation, as it was 

 called, and oiganic remains had been described from various 

 parts of it ; but to two British geologists was reserved the honour 

 of bringing order out of this hitherlo confused group of strata, 

 and establishing on stratigraphicalnnd palaeontological 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 1S31 and 1832, Sedgwick 

 made a careful section of the rocks of North Wales from the 

 Menai Strait across the range of Snowdon to the Bcrwyn 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 Llanbens 

 slates and the overlying Harlech grits), and showed that the 

 fossiliferous strata of Snowdon occupy a synclinal, and are 

 Etratigraphically several thousand feet above the horizon of the 

 latter. Following up this investigation in 1832, he estatilished 

 the great Merioneth anticUnal, which brings up the lower rocks 

 on the south-east side o: Snosvdon, and is the key to the struc- 

 ture of North Wales. From these as a base, he constructed a 

 section along the line alrer-dy indicated, over Gieat Ateni^to the 

 Bala limestone, the whole forming an ascending series of enormous 

 thickness. This limestone in the Berwyn hUls is overlaid by 

 many thousand feet of strata as we proceed eastward along tlie 

 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, underlain 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 Oxfordin 1832 ; butonly a brief and impc feet 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. and 

 D.'i'hilos. Mag. (1S54) IV., viii. 495). 



Meanwhile in the same year, 1S31, Murchison began the ex- 

 amination 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 characterised by peculiar fossils. These lorma- 

 tions were moreover traced by him to the south-westward across 

 the counties of Brecon and Caermarthen ; thus forming a belt ol 

 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 labours 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 

 lamed asiollowsindescending order :— (i) Ludlow, (2) Wenlock, 

 consJituting together an upper group ; (3) Caradoc, (4) Llandeilo 

 (or Builth), forming a lower group. The Llandeilo formation, 

 according to him, was underlain by what he called the Longinyml 

 and Gwastadeu rocks. The non-fossiliferous strata of the Long- 



• Reprinted from advance sheets of the Canadian Naturalist. 



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, 1S35, 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 north- 

 west, and appeared to be widely spread in Wales. Knowing that 

 Sedgwick had long been engaged in the siu.ly of these rocks, 

 Murchison, as he tells us, urged him to give them a British geo- 

 graphical name. Sedgwick accordingly proposed for this great 

 series of Wrdsh 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. Geok Snc. iii., 641.) This was almost 

 simultaneous with the giving of the name of Silurian ; for in 

 AuCTUst 1835, Sedgwick and Murchison ma<le communications 

 to the British Association at Dublin on Cambrian and Sdurian 

 Rocks. These, in the Volume of Proceedings (pp. 59, 60) appear 

 as a joint paper, though from the text they would seem to have 

 been separate. Sedgwick then described the Cambrian rocks of 

 North Wales as including three divisions: I. The Upper Cam- 

 brian, 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 Cam- 

 brian, 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 sub- 

 ordinate beds of serpentine and granular limestone ; the whole 

 without organic remains. 



These crystalline rocks were, however, soon afterwards ex- 

 cluded by him from the Cambrian series ; lor in 1838 (Proc. CSeol. 

 Soc. ii., 679) Sedgwick describes further the section from the 

 Menai Stiait 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 Sedg- 

 wick 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 Llanberis roofing-slates and the 

 Harlech grits or Barmouth sandstones ; and the Festmiog group, 

 which included the Lingula Hags and the succeeding Trcmadoc 

 slates. 



In the communication of Murchison to the same Dublin meet- 

 ing, in August 1835, he repeated the descriotion of the four 

 formations to which he had just given the name of Silurian ; 

 which were, in descending order, Ludlow and Wenlock (Upper 

 Si'urian), 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 slay 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 to- 

 gether the principal sections of these rocks from Caermarthenshire 

 10 Denbighshire. The gre.ter part of this region was unknown 

 to Sedgwick, but had already been studied by Murchison, who 

 interpreted the sections to his companion in conformity with the 

 scheme already given ; according to which the beds of the Llan- 



