Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/347

Rh sILUR1A.'.] GEOLOGY 333 with protrusions of granite, sycnite, and other crystalline rocks. I the base of this great series of rocks masses of white quartz-rock Towards the close of the long period represented by the Skiddaw slates, volcanic action manifested itself, first by intermittent showers of ashes and streams of lava which were interstratified with the ordinary marine sediment, and then by a more powerful and continuous series of explosions, whereby a huge volcanic mountain I or group of cones was piled up above the sea-level. The length of time occupied by this volcanic episode in Cuinbrian geology may be inferred from the fact that all the Llandeilo and nearly all the B-.1la beds are absent here. The volcanic island slowly sank into a sea where Bala organisms ﬂourished. Among these we ﬁnd siieh familiar B-ala species as Favosites ﬁbrosa, Iicliolitcs 2'-izte7'sti7Lctus, Cg/bale verrzocosa, Lcptoena se-ricca, 01-this Actmiiw, 0. b1_'forata., U. caligrmmna, 0. clegantula, 0. porcata, and Strophomcna -rhom- bozllalis. These organisms and their associates gathered on the submerged ﬂanks of the sinking volcano into a bed of liiiicstone— the Coiiistoii linicstone———which can still be traced for many miles through the Vcstnioreland hills, as the Bala limestone which it re- presents can be followed through the volcanic tracts of North Vales. The Coiiistoii limestone is covered by certain ﬂags and grits which from their organic remains are referred to the Upper Silurian series. In the South of Scotland, according to the detailed researches of the Geological Survey, the Lower Silurian formations are represented by the subjoined groups of strata in descending order :— Siiidstoiics and conglomerates, Girvan valley ................................. .. 3 Conglomerates, grits, shales, and len-1 tieular bands of limestone, Peebles- shire, Duinfriesshire, S. V. Ayr- shire, sometimes 2000 ft. .......... ..J Carsphairn group, coarse pebbly grits] and greywacke, 1200 ft ..... ...... .. Upper Black Shale, with graptolites, 550 ft. .................................. .. Lowtlier group, olive, grey, and blue shales, and sandstones, 4000 ft. Dalvcen group, greywacke and shale, with band of ﬁne conglomerate, 3500 ft ................................... .. Queeusberry group, massive grey- wackes and grits, with occasional = Llandovery. I} =Caradoc or Bala. I } =Llaiideilo (14 000 ft.) conglomerate bands and some shales, 4500 ft.. ...................... .. Lower or Motfat Black Shale group, 200-400 ft. ........................... .. Ardwcll group, brown ﬂags, grey- waekes, and shales, sometimes pur- plish and red; base not seen ...... As a whole these strata are singularly barren of organic remains. Most of the fossils which the Llandeilo groups contain lie in the bands of dark anthracitic shale which have been traced across nearly the whole breadth of the country. These shalcs are crowded with graptolites of recognizable Llandeilo forms, Climacograplus teretiuscuilus, D-iplograptus p7'z'stz's, and G-raptolitlzus Sagittarius being particularly abundant. Crustacea are exceedingly iare, but two phyllopods, Discinocmis I)’rowiu'mm and Pcltocharis aptycho1'des_, occur; while from Diinifricssliire two obscure trilobites are referred doubtful] y to Encrinurus and Phacops. The vast thickness of sandy, gritty, and shaly iiiifossiliferous strata is the distinguishing feature of the Lower Silurian series in the south of Scotland. The Caradoc or Bala group lies uneonforinably upon the upper parts of the Llandeilo rocks. It contains in the eastern districts some cal- carcoiis conglomerates which here and there swell out into local masses of limestone. In the south-west of Ayrshire the limestones attain considerable dimensions. In these calcareous bands numerous (fai-atloc species have been found, among them Chcinc-rus gclasinos-us, Ems)-imu'us pzmctatus, with species of Illcenus and Asaphus, Orthis calligrm/znia, 0. cozzﬁitis, Leptwna scricca, Maclurca, and such corals as IIcZz'0lz'tcs, Fwvositcs, Omphg/ma, and ;S‘t-rephodes. In the south—west of Ayrshire certain shales and sandstones full of Caradoc fossils are overlaid with sandstones, shales, and conglomerates con- taining Pcnmmcrus oblongus, Atrypa hcinisphcr-z'ca, ﬂferistclla angust1_'frons, Lichas lmmtzts, Pctraia clongata, JVz'dulz'tes famts, and numerous other fossils which indicate the horizon of the Llandovcry rocks. The Highlands of Scotland consist mainly of crystalline rocks— giieiss, mica-schist, clilorite-schist, clay-slate, quartz-rock, schistose ﬂagstoiie, and many others, often much invaded by granite and other intrusive masses. It was at one time supposed that these rocks all belonged to the so-called 'priinary or primitive series, older than any of the fossiliferous systems. But the discovery by Mr C. V. Peach, already referred to, that recognizable fossils occur in the limestone of Durness in Sutherlandshire, led Murchison to infer that the whole of the overlying gneissose and schistose masses are really metamorphosed Lower Silurian iocks—a generalization which has been completely conﬁrmed by subsequent investigation. At ‘ are found lying with a marked uneonforinability upon the red ' sandstones described in a previous page. These quartzose beds are merely hardened and somewhat metamorphosed sandstones; they still show their original false-beddiiig, and the casts of sea—weeds and worni-burrows. They contain a band of limestone which in Assynt swells out to a thickness of 1000 feet or more, and can be traced almost continuously from the Kyles of Skye to the north coast of Sutherlandsliire. Over these strata, in perfect conformable sequence, and with a complete lithological gradation, come quartzose tlagstones dipping like the rocks below at gentle angles to the south- east. They become more schistose and crumpled as they are traced upwards, until, after a thickness of several thousand feet has been passed over, they begin to iindulate in steep folds and pass into the ordinary schistose rocks which cover so much of the Highlands. The gradation from the comparatively unaltered lower quartz-rocks and limestones on the west to the intensely crumpled crystalline upper scliists and ﬂagstones on the east can be followed step by step in numerous tine natural sections from the nortli_of Sutherland to the Kyles of Skye. The proof is thus complete that a vast mass of seliists and other crystalline rocks overlies fossiliferous limestones in the Scottish llighlands. It therefore becomes of the utmost importance to determine the geological horizon of the fossils in the limestones. This was done by the late Mr Salter, who declared his conviction that they were unequivocally Lower Silurian, and bore a most remarkable resemblance to a group of fossils from the Lower Silurian rocks of North America. Five of the species he regarded as identical with known Ainerican forms (0rthoceras a-rcuoliratum, Hall; Orthis striatula, Eininons; Ophilcta compacta, Salt. ; Mur- chisonia gracilis, Hall; M. bellicincta, Hall), 4 as representative, 3 doubtful, and 1 new genus, found also in Canada. “That this truly North American assemblage,” he remarks, “ should be found in the extreme north of Scotland on the same parallel as the Caiiadiaii,—-that species of illacl-zu'ca and Rapkistoma, resembling those of the St Lawrence basin, and Orthoce-ram, bearing large siphuiicles like those of North America, Scandinavia, and Russia, should occur in Scotland and yet be scarcely known further south, is at least suggestive of a geographical distribution—pcrhaps even of cliiiiatal conditions—not very unlike that of more modern times.”1 From this palaeontological decision it follows that the overlying schistose series of the Scottish Highlands is a mass of metainorpliosed Silurian strata. Examined in detail they show very unequal and sporadic metainorphisni. Some poitions are scarcely more changed than the ordinary greywackes and shalcs of unaltered districts. F alse-beddiiig, pebble-beds, and other common features of sedimentation occur abundantly throughout the whole vast series of scliists. Here and there the metamorphism has be- come extreme, the rocks passing into coarsely crystalline schists full of garnets, with bands of liornblende—roek, actinolite-schist, and other metamorphic products, and passing even into granitic giieiss and true granite. N o more convincing proof could be ob- tained that vast masses of schist do not necessarily belong to an azoic period of the eartli’s history, but may have been produced by the alteration of previously existing sediments. It not necessary to believe that the sediments so altered were in all cases mere ordinary marine sand and mud. The white quartz-rocks were no doubt at one time pure white siliceous sand, the rounded grains of which can still be readily detected in them. The quartzose ﬂagstoncs were stratified sand with thin partings of clay or mud. The clay slates were evidently thick accumulations of mud. But the rocks containing a marked percentage of mag- nesia, such as clilorite-slate, actinolite—scliist, hornbleiide-rock, &c., may have resulted from the alteration of volcanic sediments and submarine lavas. The evidence from Cumberland and 'ales proves how voluminous and long continued were the volcanic eruptions of the Lower Silurian period in Britain. The abundant ditfusion of volcanic detritus over the present sea-bottom is now well known. The “ Challenger” researches have also shown us that, besides the glauconite previously known to be deposited from sea-water in the chambers of foraiiiinifera and other dead organisms on the ocean- bottoni, true magnesiaii silicates are now in the process of elimina- tion from sea-water in some of the abysses ot' the ocean. It is quite possible therefore that some of the rocks of the metamorphic series rich in niagiiesiau silicates may have arisen from the altera- tion of volcanic tufts or submarine lavas, and that others may owe their distinctive composition to original chemical precipitation, as ably contended by Sterry Hunt, though their present crystalline structure must be regarded as a part of the general inetainorphisni by which the whole of the Lower Silurian rocks of the Highlands have been affected. _ In the south-east of Ireland, grey, greenish, and purple grits, and grey and dark shales, lie uncomformably upon the Cam rian rocks, and contain a few fossils of Llandeilo age. They present interstratiﬁed beds of tuff and felsitic lavas indicating contem- poraneous volcanic action. In the north-east of the island a broad 1 Quart. Jour. Geol. Soc., xx. 381.