SILURIAN SYSTEM 



459 



tracts in Scandinavia and the basin of the Baltic, 

 and probably continue eastwards under the great 

 plains of northern Russia, for they crop up aloii" 

 the flanks of the Ural Mountains. In middle and 

 southern Europe they rise again and again to the 

 surface in mountain-regions, from which it may be 

 inferred that they underlie vast areas in the sur- 

 rounding low grounds. In North America Silurian 

 rocks extend westwards from the mouth of the St 

 Lawrence by the great lakes into the far north- 

 west, and south-west by the Alleghanies into 

 Alabama. As more or less Isolated areas rise to 

 the surface in the interior of the continent, it is 

 probable that Silurian strata, concealed under 

 younger formations, extend throughout the interior 

 continental basin ; and they reappear in the 

 Rocky Mountains. The system has been recog- 

 nised as entering into the formation of the Cor- 

 dilleras of South America. Silurian strata have 

 also been detected in the Himalayas and other 

 parts of Asia, in Australia, and New Zealand. 



The Silurian rocks which occur in mountainous 

 areas are usually much indurated and dip at high 

 angles, being frequently much contorted and dis- 

 located. In the Highlands of Scotland and in 

 Norway they have even been subjected to such 

 disturbance that they have over wide areas acquired 

 a more or less schistose or foliated character. In 

 other low-lying regions, however, as in Russia, 

 the strata are not indurated, and occur in hori- 

 zontal or gently inclined positions. 



Life of the Period. Plant-remains consist almost 

 exclusively of seaweeds, only a few traces of land- 

 plants having been met with. These indicate a 

 crvptogamic flora ferns and lycopodiaceous trees. 

 The lower forms of animal life were represented 

 chiefly by sponges ( Amphispongia, Astylospongia, 

 &c. ). Amongst the Coelenterata were numerous 

 forms of graptolites and corals. The former are 

 eminently characteristic of the Silurian, and abound 

 in certain thin bands of black shale. Those with 

 two rows of cells are mostly confined to the Lower 

 Silurian, while the single-rowed graptolites are 

 upon the whole most common in the Upper Silurian. 

 Corals abounded certain limestones appearing to 

 be almost wholly made up of their remains. Some 

 common or characteristic rugose forms were Om- 

 phyma, Zaphrentis, Cyathophyllum, &c. Promi- 

 nent tabulate forms were Halysitex, Favosites, &c. 

 The living Alcyonarian corals (Heliopora) were 

 represented by Heliolites. Crinoids or sea-lilies 

 were very numerous their jointed stems and arms 

 entering largely into the composition of many of 

 the limestones. Cyxtidi-ans, which attained their 

 ma v i inn in development in Silurian times, star-fishes, 

 and brittle-stars appear to have been less abundant. 

 Annelid-tracks are common on the surfaces of lied*, 

 and the filled-up burrows of tea-worms frequently 

 occur. Now and again jaws of annelids are like- 

 wise met with, ami occasionally their tubular cases 

 <Serpulites, Spirorbis) are seen attached to shells, 

 corals, &c. A inong the most characteristic Silurian 

 fossils were the Trilobites, which made their first 

 appearance in Cambrian was and reached their 

 greatest development in the Silurian age. They 

 lived on in much diminished numlxTs through the 

 Devonian and Carlxtniferous periods, and l>ecame 

 finally extinct in Permian times. Some of the 

 more widely dfetlibnted Silurian forms are Ogygia, 

 Trinuclens, Asaphns, llltenns, Phacops, Calymene, 

 &c. Other remarkable Arthropods were the Eurvp- 

 terids (an pxtinct order allied to the existing king- 

 crabs). One of these (Pterygotus) was seven or 

 eight feet in length. Ostracods (Beyrichia) and 

 PnyllopiMls ( ( Vratiocaris ) likewise occur in Silurian 

 rocks. Here also we meet with the first scorpion 

 ( Pal.Tophoneiift) and the earliest insect (Palmo- 

 Itlatt iria, a form of cockroach ). Various lace-corals 



(Fenestella) and other Polyzoa occur, but by far 

 the most abundant forms amongst Molluscoids 

 were the Brachiopods. Characteristic types are 

 Discina, Orthis, Leptsena, Pentamerus, Rhynchon- 

 ella, Strophomena. Lamellibranchs were much less 

 numerous Modiolopsis, Ctenodonta, and Ortho- 

 nota are examples. Amongst Gasteropods the more 

 common genera are Euomphalus, Bellerophon, Holo- 

 pella, Murchisonia, and Pleurotomaria. The Ceph- 

 alopods are well represented by straight and coiled 

 chambered cells : amongst the former is Orthoceras, 

 many species of which are known ; the latter show 

 such types as Cyrtoceras, Lituites, Phragmoceras, 

 Nautilus, &c. Vertebrates make their earliest 

 known appearance in the Upper Silurian. The 

 remains consist of bony bucklers or head-shields 

 of ganoid fishes ( Pteraspis, Cephalaspis), the defen- 

 sive spines of some cestraciont, and fragments of 

 shagreen-like skin and plates. 



Physical Conditions. The Silurian strata appear 

 to have been deposited chiefly in shallow seas, 

 which here and there, however, may have been 

 moderately deep. No certain indications of true 

 oceanic conditions have yet l>een met with. During 

 the formation of the Lower Silurian strata the 

 entire area of the British Islands, with the excep- 

 tion, perhaps, of some of the Arclnran tracts of the 

 north-west, appears to have been under water. At 

 this time active volcanoes, forming groups of islets, 

 were scattered over the area of what is now North 

 and South Wales, the south-east of Ireland, and 

 southern Ayrshire. Considerable earth-movements 

 affected the British area at the close of the Lower 

 Silurian period the l>ed of the sea being here and 

 there elevated, so that islands of considerable size 

 came into existence at the beginning of the Upper 

 Silurian period. Eventually, however, a movement 

 of depression supervened, and the islands referred 

 to were submerged and gradually buried under the 

 sedimentary accumulations of the Upper Silurian 

 sea. Judging from the geographical distribution 

 of the Silurian strata, we are led to conclude that 

 enormous areas of what are now our continents 

 were during the accumulation of those rocks over- 

 flowed by shallow seas. The main land-masses of 

 the period seem to have been grouped chiefly in 

 boreal regions and were composed essentially of 

 Architan rocks. In Europe and North America 

 alike the old land-surface lay towards the north, 

 but the great continental ridges may have risen 

 here and there to the surface in other places so 

 as to form grou|>s of islands, as in Bavaria and 

 Hohemia in Europe, ami in the region of the 

 Colorado and Park ranges in America. 



The temperature of the seas was such as to allow 

 of the migration of closely allied and apparently 

 identical species of molluscs, crustaceans, txc. over 

 vast regions. In arctic lands fossils occur which 

 are met with likewise in the Silurian rocks of Wales ; 

 more than this, many Silurian species were truly 

 cosmopolitan, ranging from the extreme north across 

 the equator to Australia. To have allowed of such 

 world-wide distribution the temperature of the seas 

 must have been singularly uniform. But while not 

 a few Silurian species were cosmopolitan, many 

 others appear to nave had a more restricted range. 

 Thus it may be inferred that, however uniform the 

 climatic conditions may have been, the uniformity 

 nevertheless was only comparative, and that even 

 in Silurian times the oceanic areas had their distinct 

 life-provinces. It is remarkable that in the Lower 

 Silurian of Scotland large erratic blocks of gneiss 

 and other crystalline rocks occur, and similar blocks 

 are met with in the Silurian strata of the Lake 

 Superior region. It is difficult to see how such 

 blocks could have been transported without the 

 agency of floating ice. See also the article* 

 MURCHISON and GEOLOGY. 



