May 1 6, 1872] 



NATURE 



53 



HISTORY OF THE NAMES CAMBRIAN AND 

 SILURIAN IN GEOLOGY* 



(Coihliidc-ii from /<7-<' 37) 



'T'lIE Lingula flags and Tremadoc slates have been made ihe 

 "^ subject of careful stratigraphical and palajjntological studies 

 by tlie Geological Survey, tire results of wliich are set forth by 

 kamsay and Salter in the third volume of the Memoirs of the 

 Geological Survey, published in 1866, and aUo, more concisely, 

 in the Anniversary Address by the former to the Geological 

 Society in 1S63. (Geol. Jour. XIX. xviii.) The I.ingula flags 

 (with the underlying Menevian. which rtseml.'les them lithologi- 

 cally) rest in apparent conformity upon the purple Harlech rocks 

 both in Pembrokeshire and in Merionethshire, where the latter 

 ap]iear on the great Merioneth anticlinal, long since pointed out 

 by .Sedgwick. The Lingula flags (nicUiding the Menevian), have 

 in this region, according to Ramsay, a thickness of about 6, oooft. 

 Above these, near Tremadoc and Festiniog, lie the Tremadoc 

 slates, which are here overlain, in apparent conformity, by llie 

 Lower Llandeilo beds. At a distance of eleven miles to the 

 north-west, however, the Tremadoc slates disappear, and the 

 Lingu'a flags are represented by only 2,000ft. of strata ; while 

 in parts of Caernarvonshire and in Anglesea the whole of the 

 Lingula flags and moreover the Lower Cambrian rocks are want- 

 ing, and the Llandeilo beds rest directly upon the ancient 

 crystalline schists. In Scotland and in Ireland, moreover, the 

 Lingula flags are wholly absent, and the Llandeilo rocks there 

 repose unconformably upon grits regarded as of Lower Cam- 

 brian age. Thus, without counting the Tremadoc slates, which 

 are a local formation unknown out of iSIerionethshire, we have 

 (including the Bangor group and Lingula flags) beneath the 

 Llandeilo, over g.oooft. of fossiliferous strata, which disappear 

 entirely in the distance of a few miles. From a careful survey 

 of all the facts, the conclusion of Ramsay is irresistible, that 

 there exist between the Lingula flags and the Llandeilo not 

 merely one, but two great stratigraphical breaks in the succession ; 

 the one between the Lingula flags and the Lower Tremadoc slates, 

 and the other between the Upper Tremadoc slates and the Lower 

 Llandeilo. 



This conclusion is confirmed by the fact that there exists at 

 each of these horizons a nearly complete palreontological break. 

 The fauna of the Tremadoc slates is, according to Salter, almost 

 entirely distinct from that of the Lingula flags, and not less 

 distinct from that of the so-called Lower Llandeilo or Arenig 

 rocks (the equivalents of the Skiddaw slates of Cumberland). 

 Hence, says Kamsiy, it is evident " that in these strata we have 

 three perfectly distinct zones of organic remains, and therefore, 

 in common terms, three distinct formations." The paheonto- 

 ■ logical evidence is thus in complete accordance with that furnished 

 by stratigraphy. We cannot leave this topic without citing the 

 conclusion of Ramsay, that "each of these two breaks neces- 

 sarily implies a lost epoch, stratigraphically quite unrepresented 

 in our area ; the life of which is only feebly represented in some 

 cases by the fossils common to the underlying and overlying 

 formation." In connection with this remark, which we conceive 

 to embody a truth of wide application, it may be said that strati- 

 graphical breaks and discordances in a geological series may, 

 a priori, be expected to occur most frequently in regions where 

 this series is represented by a large thickness of strata. The ac- 

 cumulation of such masses implies great movements of sub- 

 sidence, which, in their nature, are limited, and are accompanied 

 by elevations in adjacent areas, from which may result, over 

 these areas, either interruptions in the process of sedimentation, 

 or the removal, by sub-aerial or sub-marine denudation, of the 

 sediments already formed. The conditions of succession and 

 distribution, it may be conceived, would be very different in a 

 region where the period corresponding to this same geological 

 series was marked by comparatively small accumulations of 

 sediment upon an ocean-floor subjected to no great movements. 



This contrast is strikingly seen between the conformable series 

 of less than 2,000 feet of strata which in Scandinavia are 

 characterised by the first three palaiozoic faunas (Cambrian and 

 Silurian) and the repeatedly broken and discordant succession of 

 more than 30,000 feet of sedimentst which in Wales are their 



• Reprinted from advance sh«:ets of the Cnitadian A'aturniisL 

 1 The Longmynd rocks in Shropshire are alone estimated at 20 000 feet ; 

 but their supposed equivalents, the Harlech rocks of Pembrokeshire, h.i\-e a 

 measured thickness of 3,300, while the Llanberis and Harlech rocks 

 together, in North Wales, equal from 4,000 to 7,000 feet, and the Lingula 

 flags and Tremadoc slates, united, about 7,000 feet. The Bala group in 



paljeonto'ogical equivalents. It must, however, be considered 

 that in regions of small accumulation where, as in Scandinavia, 

 the formations are thin, there may be lost or unrepresented zoo- 

 logical epochs whoe place in the series is maiked by no strati- 

 graphical break. In such comparatively stable regions, move- 

 ments of the surface sufficient to cause the exclusion, or the dis- 

 appearance by removal, of the small thickness of sirata corre- 

 sponding to an epoch, may take place without any conspicuous 

 marks of stratigraphical discordance. 



The attempt to establish geological divisions or horizons upon 

 stratigraphical or palreontological brtaks must always prove 

 fallacious. From the nature of things, these, whether due 10 

 non-deposition or to subsequent removal of deposits, must be 

 local ; and we can say confidently that there exists no break in 

 life or in sedimentation which is not somewhere filled up and 

 represented by a continuous and conformable succession. While 

 we may define one period as characterised by the presence of a 

 certain fauna, which, in a succeeding epoch, is replaced by a 

 diflerent one, ihere will always be found, in some parts of their 

 geographical distribution, a region where the two faunas com- 

 mingle, and where the gradual disappearance of the old before 

 the new may be studied. The division of our stratified rocks 

 into systems is therefore unphilosophical, if we assign any defi- 

 nite or precise boundaries or limitations to the.=e. It was long 

 since said by Sedgwick with regard to the whole succession of 

 life through geologic time — that all belongs to one great syslctna 

 nali<r,c. (I'hilos. Mag. IV. viii. 359 ) 



We have already noticed that Barrande, as early as 1S52, gave 

 the name of Primordial Silurian to the rocks which, in Bohemia, 

 were marked by the first fauna ; although he, at the same time, 

 recognised this as distinct from and older than the second fauna, 

 discovered in the Llandeilo rocks, which Marchison had de- 

 clared to represent the dawn of organic life. Into the reasons 

 which led Barrande to include the rocks of the first, second, and 

 third fauna in one Silurian system (a view which was at once 

 adopted by the British Geological Survey and by Murchison 

 himseli), it is not our province to inquire ; but we desire to call 

 attention to the fact that the latter, by his own principles, was 

 bound to reject such a classification. In his address be'ore the 

 Geological Society in 1S42 (already quoted in the fir^t part of 

 this paper) he declared that the discussion as to the value of the 

 term Cambrian involved the question " whether there was any 

 type of fossils in the mass of the Cambrian rocks different from 

 those of the Lower Silurian series. If the appeal to nature 

 should be answered in the negative, then it was clear that the 

 Lower Silurian type must be considered the true base of what I 

 had named the protozoic rocks ; but if characteristic new forms 

 were discovered, then would the Cambrian rocks, whose place 

 was so well established in the descending series, have also their 

 own fauna, and the palaeozoic base would necessarily be removed 

 to a lower horizon." 



In the event of no distinct fauna being found in the Cambrian 

 series, it was declared that " the terra Cambrian must cease to be 

 used in zoological classification, it being in that sense synony- 

 mous with Lower Silurian." (Proc. Geol. Soc. iii. 641 ct scq.) 

 That such had been the result of paLx-ontological inquiry Mur- 

 chison then proceeded to show. Inasmuch as the only portion 

 of Sedgwick's Cambrian which was then known to be fossilife- 

 rous, was really above and not below the Llandeilo rocks, which 

 Murchison had taken for the base of his Lower Silurian, his 

 reasoning with regard to the Cambrian nomenclature, based on a 

 false datum, was itself fallacious ; and it might have been ex- 

 pected that when the Government surveyors had shown his 

 stratigraphical error, Murchison would have rendered justice to 

 the nomenclature of Sedgwick. But when, still later, a farther 

 " appeal to nature " led to the discovery of " characteristic new 

 forms," and established the existence of a "type of fossils in the 

 mass of the Cambrian rocks different from those of the Lower 

 Silurian series," Murchison was bound by his own principles to 

 recognise the name of Cambrian for the great Festiniog group, 

 with its primordial fauna, even though Barrande and the 

 Government surveyors should unite in calling it Primordial 

 Silurian. 



He however chose the opposite course, and now attempted to 

 claim for the Silurian system the whole of the Middle Cambrian 

 or Festiniog group of Sedgwick, including the Tremadoc slates 



the Berwyns e.vceeds 12,000 feet, and the proper Silurian, from the base 

 of the Upper Llandovery or May Hill sandstone, attains from 5,000 to 

 6 000 feet ; 50 that the aggregate of 30,000 feet may be considered below 

 the trutli. (Mem. Geol. Survey, iii. pari 2, pages.72, 222, and Siluria, 

 4th ed. 1S5.) 



