Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/351

SILL'RIAN.] by a series of grits or sandstones which in some places are at least 3000 feet thick. These are covered by and pass laterally into hard shales, which are believed to represent parts of the true Venlock group, perhaps even some por- tion of the Ludlow rocks. It is evident, however, that in spite of the wide extent over which these Silurian rocks of North Wales are spread, and the great thickness which they attain, they do not present an adequate stratigraphical equivalent for the complete succession in the original Silurian district. Instead of passing up coiiformably into the base of the Old Red Sandstone, as at Ludlow, they are covered by that formation unconformably. In fact they have been upturned, crumpled, faulted, and cleaved before the deposition of those portions of the Old Red Sandstone which lie upon them. place in Dcnbiglisliire when, so far as the evidence goes, there was entire quiescence in the Shropshire district ; yet the distance between the two areas was not more than about 60 miles. These subterranean movements were doubtless the precursors of those more widely extended 11 heavals which converted the floor of the Silurian sea into P' f'l lb' ' l'ltl OldPdSd a series 0 iso atet asins, in w iici ie C .‘an stone was laid down. In Vestmoreland and Cumberland a vast mass of hard slates, grits, and flags was identified by Seclgwick as of Upper Silurian age. These form the varied ranges of hills in the southern part of the lake district from near Shap to Du-.ldon mouth. The following are the local subdivisions with the conjectural equivalents in Siluria} { Flagrry beds, with lamclli- __ .- _ , Hay Fell and | l)l'iTll(3llS abundant ........... .. _('I) 'h1e'5t0mS' Kirkby Moor ihlassive greenish and greysand- { Upper Lud_ Flags ........ .. stones, with bands of fossils, = low Ilolopclla abundant ..... .. { Calcareous beds, with 1i’hyncli.o- _ Aymestry n/zlla navicula abundant . _ Limestone. Bannisd-ale Saiidstonc and shale, with star- _ Lower Lud- Slates ....... .. fish ............................ .. _ low. D-.1rk blue flags and grits of _ UpperVen- great thickness .............. .. I‘ lock. { Flags and greywacke (Ortho- | ccras siobiuuliilrttum, 0. an- 4 gIt'l((,tl'(,'))L,_'0)'(l[)t0lit]L1(.5‘ Etc- ' Z1233:-?‘iiiI:;.;f?.’.S’§:§3f’.fifZ{f:i?5?;;= < L of 4000 feet ................... .. f 0‘ ' Dark grey coarse flags (Car- (liola z'iLtcn-upta, 0;-tlioccras subimclzolatum), 1000 feet J Coniston Limestone (Lower Silurian) ................ .. = S Cmmloc or ( Pialzl. Iii the northern part of the Lake district a great anticlinal fold takes place. The Skiddaw slates arch over and are succeeded by the base of the volcanic series above described. But before more than a small portion of that series has appeared the whole Silurian area is overlapped unconform- ably by the Carboniferous Limestone series. It is necessary to ci'oss the broad plains of Cumberland and the south of Dunifries.<liii'e before Silurian rocks are again met with. In this intervening tract a syiiclinal fold must lie, for along the southern base of the uplands of the south of Scotland a belt of Upper Silurian rocks, dipping on the whole to the south-east, can be traced from the heart of the Cheviot Hills to lLl1(3l1e').(ll{1l1(lS of Wigtownshire. These rocks must reach a thickness of several thousanrl feet, but their top is nowhere seen. They repose on some of the older parts of the Llandeilo series, with so close a coincidence of dip and strike that no decided unconforinability has yet been traced betzveen them. They consist essentially of shales, with a. Coniston Grits Couiston Flags { ‘The arrangement and thicknesses here given are those in the K cndal district as mapped by Mr Aveline and Mr Hughes in the course rig tll€ Geological Survey (Sheet 98, S.E., E.q2lanatz'on, pp. 6-13, 72,. GEOLOGY These great physical changes took ‘ 337 considerable proportion of greywacke bands towards the base. At different horizons they contain lCnl}lClll.1l‘ bands of a calcareous pebbly grit. But their most characteristic feature, and one which at once distinguishes them locally from the adjoining Lower Silurian rocks, is the occurrence of a nearly black, highly fissile shale, composed of layers in most cases as thin as ordinary writing paper and usually crowded with graptolites. These peculiar bands occur throughout the whole series of rocks from bottom to top. They are sometimes so thin that 20 or 30 seams or ribs, each finely fissile, may be seen intercalatcd within the space of an inch of the ordinary shale or greywacke. Occasionally they form zones 80 to 100 feet thick, consisting entirely of finely leaved graptolitic shales. As a whole these Lpper Silurian strata resemble lithologically the correspoiid- ing series in lVestmoreland, though here and there they assume the character of mudstones not unlike those of Shropshire. The abundant fossils in them are simple l graptolites ((}')'aptolit/ms iS'cdg-zoic/.-[i', G. Becki, G. I"le77ziii_(/ii, G’. colonus, G’. G'riestoneizsis, Retiolites 6'eim't:icmus, &c.). Orthoceratites come next in point of numbers (Ort/zocerus annulutmn, 0. teimicinctzmi, &c.). In some of the shales crustacean fragments are numerous. They include large pieces of the carapace of D2'ct_z/ocai-is, with remains of 1’terygotus and Ceratiocaris. The pebbly grits contain I’etrm'a and crinoid stems. In the south of Kirkcudbright certain liniestoiies and conglomerates iiitercalated among these shales have yielded a more varied fauna, having on the whole a decidedly Wenlock character. It includes Fcwositcs, Cctteifiporct, I363,/ric/iia tuberculata, I’}uLcop.s caudal-us, Jleristclla, Lepttena sericect, Ah‘;/pa 7'et2'cularz'.,~', Stropliomeiaa imbre.r, 1l[urcliisom'a-, Orthoceras tcnuz'cz'izc(um, &c. - It is impossible in the south of Scotland to separate the Upper Silurian rocks into Wenlock and Ludlow groups. i On the whole these rocks seem to be representative mainly of the older half of the Upper Silurian formations. They are covered uncoiiformably by Lower Old Red Sandstone and later formations. In the counties of Edinburgh and Lanark, however, the base of the Lower Old Red Sandstone is found to graduate downward into a thick series of brown, olive, and grey shales, sandstones, and grits, containing undoubted Ludlow fossils. It is deserving of remark also that the peculiar lithological type so characteristic of the strata in the original Silurian area reappears in the centre of Scotland, many of the concretionary brown shales and olive-coloured mudstones being undistingiiisliable from those in the typical sections at Ludlow. Some of these beds are crowded with fossils. Among the most character- 7 istic are Leptccna transve7'salis, Ortlionota amygdalimt, . Platysc/u'sma lielicites, Beyricliia Kloecleni, Ort/zoceras Jlaclarem, with many crustaceans of the genera Ceratiocarzle, E'ur_2/pterus, Pterg/_r/otus, Slz'77zo7zia, and Slylonmws. In the Pentland Hills these strata are estimated to attain a tliick— ness of 3500 to 4000 feet, but their base is nowhere reached; in Lanarkshire they are at least as thick. Their lower portions may represent some of the higher parts of the Venlock group. Ireland furnishes some interesting evidence regarding the geographical changes in the west of Europe between the- close of the Lower Silurian and the beginning of the Upper Silurian period. It has already been pointed out that the metamorphosed Lower Silurian rocks of the Scottish High- lands are prolonged into the north of Ireland, whence they range south-westwards to Galway Bay. In the picturesque tract between Loch Mask and the month of Killary harbour these metamorphosed rocks are unconformably overlaid by ' masses of sandstones, conglomerates, and shales more than 7000 feet thick, and containing Llandovery and lVenlock fossils with a mixture of Caradoc forms. X. '— 43

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