Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/352

Rh .— 38 of the greatly metamorphosed Lower Silurian platform, por- tions are to be found still little altered and full of fossils. ’l‘lie overlying Upper Silurian strata have not been 1iieta- niorpliosed, but contain pebbles of the altered rocks on the upturned edges of which they lie. It is evident therefore, as 311' Iliill has remarked, that the mctainorphisin must have occurred between the close of the Lower and the coin- meneement of the Upper Silurian period.‘ In connexion with this question it should be remarked that abundant , volcanic activity accoinpanietl the deposit of these Fpper Silurian rocks in the west of Ireland, successive sheets of liva (curite) mid beds of tiiff forming conspicuous bands -.::noiig the stratified rocks, and reaching a collective tliick- iiess of 800 feet and upwards. Between Brandon Head and Dingle Bay :1 thick mass of strata on the coast, must, from the comparatively few fossils obtained from it, be held to represent Upper Silurian formations. C0.'Tl.‘B.'T.L EL'i:orE.—The broad hollow which, run- ning from the mouth of the English Channel across the plains of northern Germany into the heart of Russia, divides the high grounds of the north and nortli-west of Europe from those of the centre and south, separates the European Silurian area. into two distinct tracts. In the northern of these we ﬁnd the Lower and Upper Silurian formations attaining an enormous development in Britain, but rapidly diminishing in thickness towards the iiortli-east, until in the south of Scandinavia and the (tulf of Finland they reach only about Eljtll of that depth. In these tracts, too, they have on the whole escaped so well from the dis- locations, crumplings, and metaiiiorphisins so conspicuous to the soutli—west, that to this day they remain over wide areas nearly as horizontal and soft as at first. In the sauthern tract Silurian rocks appear only here and there from amidst later formations, and almost everywhere pre- sent proofs of intense subterranean inoveinent. In the south of Scandinavia (Christiania, ll_jiiscn See, Malmii, llothland) the Lower and Upper Silurian rocks attain a united thickness of not more than about 1200 feet, yet are said to contain representatives of all the leading subdivisions of the British series. 'l‘lic following table exhibits the Silurian succession in the south of Norway and Swedeii, with the supposed English cquivalents:— Sandy beds, with Ptcrinca 7'ctro_ﬂc.r(z, 1274,1171- rhonrlla izucula, Orthonota 7'ctu.s-(1, 1}r_2/-} zlfpper Ludlow. 7'ichin lubcrculala. S, Gothland . .......... .. Upper Malinii limestone . .................... Upper Graptolite niarls, with (:'r(q)lolz'lIzus priodon (Licdcnse) abundant ................... .. Lower lI:ilini} or upper Orthoceratite limestone, T = Lower Ludlow. with large Orlleoccrata having central siphuncles ..... ................................ _. J lincrinital so.-hists with orthoeeratitcs and Gam- plmcrras p_2/riformr; ............................... .. __ - _ f ‘oral liinv-stone (0:npIig/nm lwrbi'21atu7n and i _“C“1Oc]" other Wenloek corals) .......................... ..} l’«-ntanierus limestone, 1’cnlamcrus Ol/l0il_//NS, I’. grolcaliis‘, .S'lr[c/.'I(mrl£nz'(c lcns, Lrpl(I_'nIz _ L} 1 _ frrmsvcrsalas, Eucriimrus ])1LIlt'l(Ill(8, &c. . . l I" ‘mi Over)‘ Lower argillaceous sclii:-its ..................... .. J Culcai-eons sandstones (containing a mixture of‘ Llandovery forms, as .lII.'rz'.slcll(L (mgu.sli_'/'rons, and many large smooth I’cnlumcri) ......... .. Caleareous and aruillaeeous lla rstoncs Orlhis cr¢lli_I/rauzrim, 0.0!:slu(lin(u'z'af)0. ])f,'(‘[’L’II, Lg»- luzna scricea, Conuluria quatlrisulcalu, Asa- jihus crprznsiis, T rimtclcus concculricus, &e. Oi-thloeeratite limestone and lower lincrinital av llStS ............................................. ..J Upper Graptolite s--hists, with Diplogrrqitusi pri-s-/is, 1). fuliuui, I}. (crcliusculus, and forms of Asnplms, Ogygia, Triilucleus, &e. Lower Orthoeeratite limestone, with Ortlioccras rliqilnr, 0. (Innululum, Lituitcs cornu-m-z'rlz's, Ilrlliis calligramma, 0. clegrmtula, Brllnro-J phon bilobatus, 8:0 ............................... .. t = Caradoc. r = Ll-andeilo. 1 Phi/sr'c(I.l Geology If Ireland, p. 22; Kinahan's 6.'c-.I4._q_i/ Q/'Irclmi(I, chap. iii.; G'eolo;'/iciil -S'urrr'y qf Lzluml, 1;‘.:-pl:_uuI(:'on of SI'¢cct.>' (76, 77. 83. and 94!. [Va STR.TIGRAI'IlI('AL. Lower Graptolite schists (with numerous grap- tolites) .............................................. .. Alum schists, 150 feet (= =l{i-giones . and ll of } -llgClll‘l)), containing the fossils enuinerated } =(.mnh_iuu. on p. 3.1 .......................................... ... Quartvite (_l"llt'bi(l.'; ................................. ..} Though the general reseiiiblziiice of the succi-ssion of fossils in Scandinavia and in Tirlttllll is singularly close, there are, as might have been anticipated, ditferciices in the range of species, some forms having appeared earlier or having survived later in the one region than in the other. Thus the I’ent«mm'us U])[OIl_(,'I(S ascends in Scandinavia into rocks full of Wenlock corals, but does not occur in the 'enlock group of Britain. On the other liand, among Scandinavian strata containing such cliai'.ictcristic=illy Lower Silurian genera of trilobites as .l.-a_pIm.s, T7'z'imr{¢2(.-, and 01/_I/5/fa, there occur organisms which in Britain are typically l'pper Silurian, such as Ortlioceras (l2'mz'd{atum and U. (lis- icuzs, two fossils of the Liidlow rocks. This is a point of considerable importance in its bearing upon the value of palzeontological evidence in correlating the formations of different countries. It shows that the order of succession found to hold good in one region cannot be rigidly applied to others, and that in such cases it is not from individual species so much as from the general facies of the fossils that we must draw geological parallels. The fiI‘>t appearance and duriition of a species have doubtless greatly varied in ditl'erent regions. In llussia .'‘ilurian rot-ks must occupy the whole vast lIl'(':ltllll (-f territory between the Baltic and the flanks of the l'r:il Moiiiitziiiis, beyond which they spread eastwards into Asia. Througliont niost of this extensive area they lie in lll'l'/.Olli:ll iiii«li.sturbcd beds, covered over and concealed from view by later formations. Along the flanks of the lirals they have been upheave_l, and placed on end or at a high angle against the central portions of that chain, and have been partially metaniorphosed into cliloi-ite-scliist, llilt':l- seliist, quai-tz—i'ock, and other crystalline masses. But along the soutlicrn margin of the Gulf of 1"inland they appear at the siii't'ai-e as soft clays, sands, and unaltered strata, which, so far as their lithologieal characters go, might be supposed to be of late Tertiary date, so little have they been (-lianged during the enormous lapse of ages since Lower Paheozoie time. The gn’-at plains between the l'ral chain on the east and the rising grounds of (it'l‘lllﬂll_' on the south-west have thus from a remote geological anticpiity l.ei-ii exempted from the terrestrial coiriigatioiis which have atl'et-tcil so much of the rest of liuropc. They have been alternately, but gr-iitly, depressed as a sea-floor, and elevated into st:-ppes or plaiiis. The following subdivisions have been cstablislicd by I". Si-limidt among the Silurian rocks of nortli-west Russia :—'-' l. QipcrSilzu-i'm1— Sandy variable limestone, with marly layers] passing into sandstone (L’c_2/)'£cIu'a tuber- culrzlu, Gramnig/.s'z'a cingztlula, ('l(0n('lrs ._ vfilestont-.-. .slriatcIlu, and iiuincrous fish l'l'l]ltllllS,J Om-lms, Upper Oesel group, yellow niarly and sonic-‘ times doloinitie strata (].’li_i/iiclmncllrl II '17- -- Ludlow. r-Arenig in part. soni, (_'/umctcs .5-lriutcllzt, I'l(([_I/.S‘I'llI's))2(I lull- citrs, 1'.'iu'3/plcrus 'rcmi'1:cs, and fish re- mains, &c.) ...................................... .. Lower Oesel group, dolomite, with marl and‘ limestone below I’/‘opera tulmlala, IIaI_2/- sitrs (lis/(ms, I203/ricl42'a I{I¢'i'(lcm’, l'.'-)zcri- 92uruspmiclalus, I’roe'lus concimms, ]|[cr1's- trlln (iunida, .S'p[)'{/cr crisgms, Lqilwntt. tr(ur.si'c2's(zl:'s, Ell07}I2)]lﬂlll8fl£7l('tll8, Url/io- crras annulutmn, &c.) ....................... .. J ll. Midrllc .S'i'lm-{rm- l’cnti1incrus-i band, with I’. clislon-us (ob-‘ Iongils), /1l1'coI[tc.s I.IIl;ccIu'1', L’r‘Ilrro,iImn (lilalalus, L’ronlcus signritus (lalicrnulu) Compact limestone and dolomite with siliceous nodules (II cl iol ilcs z'nlc7'sl incl us, I ‘I [led {cl ya scalpcllum, SlropIim72c1m pcclcn, 0rlIiisI¢_1/- Lrz'¢l(I., I’c:2lmncrus linguzfcr, ].I'pc)-zlilia mrrrghirzlrz) .................................... . . I’entain(-rus band, limestone, and dolomite, with ]’»ulumI‘rus Inn-mlis, &c. ............ ..J > = “iclllocli. » -—— Llaiidovery. '-' I'iiIw'sm-Iimigrw iibrr (Hr .'i'lim'sr_'Izc I"urmali'o-n rm: Estlancl, .'ur:_l- I.[r_7cIm7, an/I U'.'.x.'l, llorpat, '15.’-8.