Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/341

Rh TABLE or Form.-'rIo.'s.] But whatever be the name chosen to designate a particu- lar group of strata, it soon comes to be used as a chronologi- cal or homotaxial term, apart altogether from the strati- graphical character of the strata to which it is applied. Thus we speak of the Chalk or Cretaceous system, and embrace under that term formations which may contain no chalk ; and we may describe as Silurian a series of strata utterly unlike in lithological characters to the formations in the typical Silurian country. I11 using these terms we unconsciously allow the idea of relative date to arise prominently before us. Hence such a word as chalk or cretaceous does not suggest so much to us the group of strata so called, as the interval of geological history which these strata represent. Ve speak of the Cretaceous, Jurassic, and Cambrian periods, and of the Cretaceous fauna, the Jurassic flora, the Cambrian trilobites, as if these adjectives denoted simply epochs of geological time. The geological record is classified into five main divisions: —(l) the Arcluean, .zoic (lifeless), or Eozoic (dawn of life) Periods; (2) the Primary or Palaeozoic (ancient life) Periods; the Secondary or Mesozoic (middle life) Periods ; (4) the Tertiary or Cainozoic (recent life); a11d (5) the Quaternary or Post-Tertiary Periods. These divi- sions are further ranged into systems, each system into formations, each formation into groups, and each group or series into single zones or horizons. The subjoined gene- ralized table exhibits the order in which the chief sub- divisions appear. 0,-dcr Qf »S'ur:cc.9si0n of the Stratiﬁed Formation of the Earl/L’s ("rust ,' Britain. Continental Europe. North America. I‘.eecnt—.lluvunn, ..lluvium. Recent or Terrace. -3 ‘_, pent. .'c. ' _ I _ 1-‘ .-. I 101-t0Ct'IlC—( 411’ Diluvium. Champlain. E; 41:12 isizs, Glacial _.f :_ dI'lﬂ- Glacial. —' I _ l'iim-c_ne—_I_'/-.4:_; .I.- l‘liocene—7‘.-gel, Dino- Sumter. I; 1'"-<7/-71'. ur_7o.’I." and tlzerziun-S:,u1¢I. § b'u_[]'u.’ '. I 3 .liocene-——Li_r/n1'tz' nf Minc-eue—Lrz'lIml'aIk, Yorktown. '_E-Z {over} TI'ac('y. Jlull, Upper Jlolas.-e. -' (C. E 0ligoccnc—I.mr.’r Jin- b.’euu &C. =_: I2occne—7'wrlz'ari'es of EoCcne’—-.Vummuli[c- Alabama. g: Ilampslzire lhtsin, Iimesloue, 1-Yysch. Lignilic. (nu! Isle of Wiglrl. Upper Senonian—('raz'e blanche 1-‘ox-llillsgroup. I :- ct tu_[f:’aII. Fpper Qua- _g _. der.m71d.-tein. F2 3 -g Cretaceous. 'l'uroni:m—1’.'¢‘i:zerl-alk. Pierre group. '5 E 5* Cenomanian—Grés rcrt. .‘ii-brara group. gt: [ Gault. licnton group. ;_, 5 ‘.7. L'0w-:1‘. .:L'0O0ﬂ]l:lll. Dakotah group. "- 3 5 _ _ Lppcr. Lpper or White J ura .h .-5; Ooluic. _ . Ialn. Jurassic rocks ap- ._-3 w, Lower. I IIuldle or Brown J um pear to be but E E _ _ ‘ (Dogger). poorly developed 3 - Liassic. I.ower or Black J ura in N. America. _‘.a (Lias). L _ _ ‘ (Upper. llhzetie beds, Keuper. ' _ Tl‘i:lS>lC. - .lu>cl1elknlk. Tl'l3.ESlC. i l.0'er. liunter. ' I r - . . Pcriman. l Dvas or ([('clIs!(-‘III, Permian. 5 _ ‘ Permian] Itotlzlicg/critics. -5 . _ ( Coal-measures. ( Terrain houiller, b'tein- Carboniferous. 5 ’- kohlen. E llillstone Grit. l 1-‘ liirzlcercr Samlstcin Sub-Carboniferous. _=: _.. ._ Carboniferous Calcaire Carbonifere,
 * ». Iusse. Grés (Ie Fon!az'ne-
 * " l.‘n1L-stone. Kolilelikalli, liulm.

o Devonian and Old lied Devonian Devonian. g Silurian. Silurian (Transition or Silurian. .5 ‘ _ fa‘-1'auu_-aclgesystem). _!:1‘l'ﬂ_ll''a'liC _and slate. and Caunbrian. Pruniuvc schists. llurouian. I-‘undamcntal gneiss. Cr-gneiss. Laurentian. GEOLOGY 327 I. AI-’.CII.*’l3A'.'. Underneath the oldest unaltered stratified and fossiliferous formations in Europe there occur masses of gneiss and other crystalline schistose rocks belonging perhaps to widely ditferent geological periods, but, from want of satisfactory means of discrimination, necessarily united provisionally in one common series. That they are separated by a vast in- terval of time from the rocks which lie upon them is shown by the strong unconformability with which theyare related to every formation of younger date than themselves. Every- where thoroughly crystalline, they are disposed in rude, crumpled, often vertical beds, out of the ruins of which the overlying formations have been partly built. ]3RITAIN.——Il1 no part of the European area are these ancient rocks better seen than in the 11orth-west of Scotland. Their position there, previously indicated by MacCulloch and Hay Cunningham, was first deﬁnitely established by Murchison, who showed that they possess a dominant strike to N .N. W., and are unconformably overlaid by all the other rocks of the Scottish Highlands. They consist of a tough massive gneiss usually hornblendic, with bands of horn- blende-rock, horublende—schist, quartz-felsite, granite, and other crystalline rocks. In two or three places they enclose bands of limestone, but neither in these nor in any other parts of their mass has the least trace of any organic struc- ture been detected. It is impossible at present to otfer any conjecture as to their probable thickness. It must be many thousand feet ; but its approximate amount, if ever ascer- tainable, will only be made out after the region where they occur has been mapped in detail. These gneisses and schists possess a massiveness and rudeness of bedding which strongly distinguishes them from all the other and younger metamorphic rocks of Britain. They form nearly the whole of the Outer Hebrides, and occupy a variable belt of the western parts of the counties of Sutherland and Ross. Murchison proposed to term them the Fundamental or Lewisian Gneiss from the isle of Lewis—the chief of the Hebrides. Afterwards he called them Laurentian, regard- ing them as the equivalent of so111e part of the great Laurentian system of Canada. In recent years Mr Hicks and others have endeavoured to show that in Wales there exist here and there protrusions of an old crystalline group of rocks from beneath the Cam- brian system, and they have described these “pre-Cambrian” masses as overlaid unconformably by younger formations, as in the north—west of Scotland. I’rofessor Ramsay, how- ever, who with his colleagues in the Geological Survey mapped the lVelsh areas in detail, contends that the sup- posed older gneiss is merely a metamorphosed portion of the Cambrian rocks. CONTINENTAL EURoPE.—On the continent of Europe numerous areas of ancient gneiss rise from under the oldest fossiliferous formations. In Scandinavia the structure of part of the country resembles that of the north—west of Scotland: the fundamental-gneiss (U-2'!/2zez'ss), covering a large area, is overlaid unconformably by red sandstones which underlie the most ancient strata containing organic remains. The gneiss and its accompanying rocks range through Finland into the north-west of Russia, reappearing in the north-east of that vast empire in Petchora Land down to the White Sea, and rising in the nucleus of the chain of the Ural Mountains, and still further south in Podolia. In Central Europe they appear as islands in the midst of more recent formations. In the midst of the Carpathian Mountains they protrude at a number of points, but westwards in the Alpine chain they rise i11 a more con- tinuous belt in the central portion of these crests, and show numerous mineralogical varieties, including protogine, mica-schist, and many other schists, as well as limestone
 * -3 Saiuistonc-.
 * ., _ C xmorian. Primordial Silurian. older Primordial Silurian