‘T ra n s itio n R o c ks a n d G ra u w a c k e'’一 T h e S ilu ria n a n d C a m b ria n System s through 150 Y ears by M ichae l B assett The terms Silurian and Cam brian were first published F orm a tiv e S tu dies in a ge o lo gi ca l sen s e in 18 3 5 . Th is a rt ic le r e vi ew s the historical background to the concept and early Sedgw ick and M urchison first m et in the w inter of 1824-25, application of these stratigraph ical division s, and w hen the latter joined the G eological Society of London, their international status after 150 years of being adm itted to m em bership on January 7th, 1825. d ev el op m en 亡。 Sedgw ick had been a m em ber since 1818 and had rapidly becom e a leading figure in the Society, w hich by then w as In tro du ction o ne o f the m a in foru m s fo r th e d iscussio n of sc ie nc e an d philosophy in Britain. A s an authoritative teacher, Sedgw ick D ublin, Ireland, w as the setting for the fifth annual m eeting soon exercised a profound influence on the younger m an, w ho of th e B ritish A ssoc iatio n fo r the A d va nc e m en t o f Sc ie nc e in w as b oth a m b itio us an d d ete rm in ed to m a ke h is m ark o n th e A ugust 1835. O n Friday, A ugust 14th, the R everend P ro- exciting geological developm ents of the early 19th C entury. fessor A da m Sedgw ick and M r. R oderick Im pey M urchison A fter initia l instru ction fro m D e an W illiam B u ck lan d o f (F igs. 1 and 2) read a com m unication to the final session of O xford U niversity, m uch of M urchison's detailed appreciation the G eology and G eography Section entitled "O n the Silurian of field observation and interpretation w as w ith Sedgw ick, and C am brian System s, exhibiting the order in w hich the follow ing the first of their geological excursions together in older Sedim entary Strata succeed each other in E ngland and July 1827. W ales." Sedgw ick read the details of the C am brian rocks as established by his research, w hilst M urchison discussed the B y the late 18201s, the succession of fossiliferous "Secondary" basis for his Silurian stratigraphy; this presentation, by the and "Tertiary" rocks of England (essentially Upper Palaeozoic see m ingly unlikely com bination of a C a m bridge U niversity and younger) w as reasonably w ell established, based on the profe ssor/clergym an and an ex-soldier turned am ateur geol- principles of m apping and faunal superposition dem onstrated ogist, w as the first announcem ent at a form al scientific first by W illiam Sm ith. T o the north and w est, how ever, lay m eeting of the term inology and relationships of the m ajor large tracts of older, supposedly poorly fossiliferous "arena- stratigraphical units that w ere to becom e accepted univer- ceous and slaty rocks" w hose succession and relationships sally as the low er and upper divisions of System rank w ithin r e m a in e d n o t w e ll u n d e rs to o d . T h e s e o ld e r ro c k s w e r e c o m - t h e L o w e r P a l a e o z o ic ro c k s . T he official proceedings of the D ublin m eeting w ere not published until 1836, but two separate reports appeared in 1835, the first being privately printed in D ublin in Septem ber (H ardy, 1835; see also M orrell and T hackray, 1981, p. 185), and the second d istrib ute d to a w ide r sc ie ntific a ud ie nc e in th e D ec e m be r issue o f T h e L on do n an d E dinburgh Philosophical M agazine and Journal of Science (Sedgw ick and M urchi- son, 1835). A lthough the form er account is relatively poorly know n, it constitutes the first form ally published record (on page 107) of the term C am brian as presented by Sedgw ick. In the case of the Silurian System , how - ever, M urchison bad pre-em pted the announcem ent at the D ublin m eeting by publishing a paper in July 1835 entitled "O n the Silurian System of R ocks" (M urchison, 1835). T hroughout their scientific careers M urchison w rote up and prom oted his results m uch m ore rapidly and in greater volum e than Sedgw ick, and although in 18 35 th e tw o w ere c lo se frie nd s and co 卜 laborators, this early publication by M urchison of "his System " w as sy m pto m at- ic o f w h at w as late r to b e co m e the w ell- know n, bitter controversy betw een the tw o 几 gu re 1 : A da m S ed gw ic k Figure 2 : Roderick Murchison m en (e.g. see T hackray 1976). (1785-1873 ) aged 47. (1792-1871 ) probably aged 55 。 Episodes, Vol. 8, N o. 4, D ecem ber 1985 2 3 1 m only referred to as "T ransition R ocks" or "Transition ture and succession of the rearion. "For a first survev. I (i.e. Series," im plying that they had form ed in a period transi- M urchison) had got the upper grauw acke, so called, into m y tional betw een the earliest chem ical or m olten phases of hands, for I had seen it in several situations far from each earth history and a tim e w hen m ore recognizably m odern other all along the South W elsh frontier, and in Shropshire faunas and floras developed w orldw ide. The old G erm an rock and H erefordshire, rising out conform ably from beneath the nam e "G rauw ack6l' (both w ith and w ithout th e C ontinental low est m em ber of the old red sandstone. M oreover, I had accent) w as also used as an approxim ate synonym of these ascertained that its different beds w ere characterized by "T ransition R ocks," since this general lithological type w as a peculiar fossils." (G eikie, 1875, pp. 183, 184). substantial com ponent. T hus, in one rem arkable first sum m er, M urchison established In his lecture syllabus at C am bridge as early as 1821, the lithological and faunal control for m ost of his later w ork. Sedgw ick had certainly begun to consider som e of the prob- Such w as the scope of his coverage, that in Septem ber 1831, le m s asso cia ted w ith th e classifica tion o f the o lde r stra tified at the inaugural m eeting of the British A ssociation for the rocks (C lark and H ughes, 1890, pp. 288-290, 533); he had been A dvancem ent of Science held in Y ork, he dem onstrated aw are of them in his native north-w est England fro m boyhood coloured m aps of the "Transition R ocks" that he considered and began to study them seriously there in 1822. It is not to be "a good broad announcem ent of a new step in B ritish difficult to assu m e that he discussed these studies m any geology" (G eikie, 1875, p. 184). 纂 Figure 3 : Ludlow and its No rman castle 一 part of F ig ur e 4 : Th e Rh in o g M o u n t a i n s i n t h e H a r l e c h D o m e Murchison 's type Silurian region in the Welsh Border- o f n o r t h - w e s t W a l e s ～ the heart of Sedgwick 's Cam- land . Upper Ludlow rocks in the foreground pass up br ian System ; the rocks ar e m o s tl y co ar s e az en it es o f i n t o D e v o n i a n O l d R e d S a n d s t o n e f a c i e s o f t h e C l e e e ar ly C am b ri an a ge . H i l l s i n t h e d i s t a n c e . tim es w ith M urchison, and by 1831 the tw o had determ ined to M eanw hile, Sedgw ick finally left C am bridge at the beginning unravel the succession in W ales and the adjacent W elsh of A ugust and proceeded to N orth W ales, w here he arrived on Borderland where the 11grauwack611 and associated slaty rocks A ugust 5th, 1831.