Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/343

Rh CAMBRI.-tN.] disappearance and extinction of graptolites and trilobites, and the abundance of its cryptogamic terrestrial ﬂora. CAMBRIAN. This name was applied by Sedgwick to the rocks of North Vales (Cambria), where he ﬁrst investigated them. Their base is there nowhere seen, so that, though they attain a great depth, some part of their total mass must be concealed from view. They pass up continuously into the base of the Silurian system. Considerable diversity of opinion has existed, and still continues, as to the line where the upper limit of the Cambrian system should be drawn. Murchison contended that this line should be placed below the strata where a trilobitic and brachiopodous fauna begins, and that these strata cannot be separated from the overly- ing Silurian system. He therefore included in the Cambrian only the barren grits and slates of the Longmynd, Harlech, and Llanberis. Sedgwick, on the other hand, insisted on carrying the line up to the base of the Upper Silurian rocks. IIe thus left these formations as alone constituting the Silurian system, and massed all the Lower Silurian in his Cambrian system. Murchison worked out the stratigraphi- cal order of succession from above, and chieﬂy by help of organic remains. He advanced from where the superposi- tion of the rocks is clear and undoubted, and for the ﬁrst time in the history of geology ascertained that the “ transi- tion—rocks” of the older geologists could be arranged into zones by means of characteristic fossils as satisfactorily as the Secondary formations had been classiﬁed in a similar manner by William Smith. Year by year, as he found his Silurian types of life descend farther and farther into lower deposits, he pushed backward the limits of his Silurian system. In this he was supported by the general consent of geologists and palaeontologists all over the world. Sedg- wick, on the other hand, attacked the problem rather from the point of stratigraphy and geological structure. Though he had collected fossils from many of the rocks of which he had made out the true order of succession in North ‘Vales, he allowed them to lie for years unexamined. lleanwhile Murchison had studied the prolongations of some of the same rocks into South Wales, and had obtained from them the abundant suite of organic remains which characterized his Lower Silurian formations. Similar fossils Were found abundantly on the continent of Europe, and in America. Naturally the classiﬁcation proposed by Murchison was adopted all over the world. As be included in his Silurian system the oldest rocks containing a distinctive fauna of trilobites and brachiopods, the earliest fossiliferous rocks were everywhere classed as Silurian, and the name Cambrian was discarded by geologists of other countries as indicative of a more ancient series of deposits not characterized by peculiar organic remains, and therefore not capable of being elsewhere satisfactorily recognized. Barrande, investigat- ing the most ancient fossiliferous rocks of Bohemia, distin- guished by the name of the “ Primordial Zone ” a group of strata underlying the Lower Silurian rocks, and containing a peculiar and characteristic suite of trilobites. He classed it, however, with the Silurian system, and Murchison adopted the term, grouping under it th) lowest dark slates which in ‘Vales and the border English counties contained some of the same early forms of life. Investigations during the last twelve years, however, chieﬂy by the late Mr Salter and Mr flicks, have brought to light a much more abundant fauna from the so-called primordial rocks of Vales than they were supposed to possess. These fossils Were found to be in large measure distinct from those in the undoubted Lower Silurian rocks. Thus the question of the proper base of the Silurian system was re-opened, and the claims of the Cambrian system to a GEOLOGY 329 great upward extension were more forcibly urged than ever. But these claims could now be urged on palaeontological evidence such as had never before been produced. Accord- ingly there has arisen a general desire among the geologists of Britain to revise the nomenclature of the older rocks. Though as yet a common accord of opinion has not been reached, there seems a strong probability that ultimately the boundary line between the Cambrian and Silurian systems. will be drawn above the primordial zone along the base of the great Arenig group or Lower Llandeilo rocks of Murchison. All his Silurian strata of older date than these rocks will be classed as Cambrian. According to this classiﬁcation, the Cambrian system, as developed in North Wales and the border English counties, consists of purple, reddish—grey, and green slates, grits, sand- stones, and conglomerates. Its true base is nowhere seen, yet even the visible mass of strata has been estimated to reacl1 the enormous thickness of 25,000 feet. By far the larger part of this vast depth of rock is unfossiliferous. Indeed it is only in some bands of the upper 6000 feet, or thereabouts, that fossils occur plentifully. By fossil evi- dence the Cambrian system may be divided into Lower and Upper, and each of these sections may be further subdivided into two groups, as in the following table :— _ 4. Tremadoc slates. Ctar‘nbrlian g Uppel‘ 3. Lingula ﬂags. o a es. _ 2. Menevian group. L0“ er’ 3 1. Harlech and Longmynd group. 1. Harlcch and Longmynd G'r0up.—This includes purple, red, and grey ﬂags, sandstones, and slates, with conglomerates. These strata attain a great thickness, estimated at 4000 feet in South Valcs, but more than 8000 in North Vales. They were formerly supposed to be nearly barren of organic remains; b11t in recent years, chieﬂy through the researches of Mr Hicks at St Davids, they have yielded a tolerably abundant fauna, consisting of 30 species. Among these are 16 species of trilobite (Pa;-ado.rz'dcs, Plutonia, Jllicrodiscus, Palmo- pygc, Agnostus, Conocoryphc), fo11r annelidcs (A7'cnic0l1'lcs), a sponge (I’7'0t0spongia), five brachiopods (Discina, Lingulella), two pteropo(ls (Thccu), &c. Many of the surfaces of the strata in some parts of this group are marked with ripples, sun-cracks, and rain- pittings as well as with trails of worn1s—indicative of shallow- water and shore-conditions of deposit. 14 of the 30 species, accord- ing to Mr Etheridge, F.R.S., pass up into the Menevian group, and 7 continue into the Lingula ﬂags. 2. Jlfcncz.-ian G'roup.—This subdivision has been proposed for a series of sandstones and shales, with dark-blue slates and ﬂags, dark—grey ﬂags and grey grits, which are seen near St David’s (Menevia), where they attain a depth of about 600 feet. They pass down conformably into the Harlech group with which, as just stated, they are connected by 14 species in common. The Menevian beds have yielded upwards of 50 species of fossils, of which 24 are conﬁned to the Menevian, while 18 pass up into the lower Lingula ﬂags. Among these the trilobites are specially prominent, Some of then) attained a great size, Pa7'ado9cz'dcs Davidis being nearly two feet long. But with these were mingled others of diminutive size. It is noteworthy also, as Mr Hicks has pointed out, that while the trilobites had attained their maximum size at this early period, they are represented among the older Cambrian rocks by genera indicative of almost every stage of development, “ from the little Agnostus with two rings in the thorax, and ﬂIic7'0dz'.9cus with four, to Erimiys with twenty-four,” while blind genera occur together with those having the largest eyes.‘ Upwards of 30 species of trilobites have been obtained from the Menevian beds, the genera Agnostus (7_species), Conocoryphc (7 species), and Pam- doxidcs being specially characteristic. Four species of sponges (Pro- tospongia.) and some annelide-tracks likewise occur. The mollusca are represented by 6 species of brachiopoda of the genera Discina, Lingulclla, and Obolclla; 5 pteropods (Theca) have been met with. The earliest entoniostraca (Entomis) and the first cystidean (Proto- cystites) yet discovered occur in the Menevian fauna. 3. Lingula Flags.—-These strata, consisting of bluish and black slates and ﬂags, with bands of grey ﬂags and sandstones, attain in some parts of Wales a thickness of more than 5000 feet. They received their name from the discovery by Mr E. Davis (1846) of vast numbers of a Lingula (Lingulclla Davisii) in some of their layers. They rest conformably upon, and pass down into, the Mcnevian beds below them, and likewise graduate into the Tre- madoc group above. They are distinguished by a. characteristic 1 Hicks, Quart. Jomvz. Geol. Soc., xxviii. 174. R. -— 42