54 



NATURE 



[Afay 1 6, 1872 



and the Lingula flags. The grounds of this assumption, as set 

 fortli in tlie successive editions of ''Siluria" from 1854 to 1S67, 

 and in various memoirs, may be inc'uded under three heads : 

 first, that the Lingula flags have been found to exist in some 

 parts of his originil Silurian region ; second, that no clearly- 

 defined base had been assigne 1 by him to his so-calied system ; 

 and third, that there are no means of drawing a line of demarc- 

 ation between these Middle Cambrian formations and the over- 

 lying Llandeilo. 



With regard to the first of these reasons, it is to be ssid that 

 the only known representatives of the Lingu'a flags in the 

 region described by Jdurchison in his " Silurian System " are the 

 black slates of Malvern, and some scanty ouiliers which, in 

 Shropshire, lie between the old Longmynd rocks and the base of 

 the Stiper-stones, The former were then (as has already been 

 shown) supposed by him to belong to the Llandeilo, or rather to 

 the passage-beds between the Llandeilo and Cambrian (liala) ; 

 whde with regard to the latter, Ramsay expressly tell us that 

 they were not originally classed with the Silurian, but have since 

 been included in it. (Mem. Geol. Sur. iii. part 2, page 9; 

 and 242, foot-note) 



The Llandeilo beds were by Murchison distinctly stated to be 

 the base of the Silurian system (Sil. Sys. 222) ; and it was further 

 declared by him that in Shropshire (unlike Caermarthen shire) 

 " there is no passage fi-om the Cambrian to the Silurian strata," 

 but a hiatus, marked by disturbances which excluded the passage- 

 Veds, and caused the Lower Silurian to rest unconformably upon 

 the Longmynd rocks. (Ibid, 256 ; .and plates 31, sections 3 and 

 6; 32,section4) Butin "Siluria" ( 1st ed. 47) the two are stated 

 to be conformable ; and in tire subsequent sections of this region, 

 made by Aveline, and published by the Geological Survey, the 

 evidences of this want of conformity do not appear. Murchison 

 at that time confounded the rocks of the Longmynd .with the 

 Cambrian (Bala) beds of Caermarthenshire and IJrecon. (.Sil. 

 Sys. 416). Hence it was that he gave the name of Cambrian to 

 the former ; and this mistake, moreover, led him to place the 

 Cambrian of Caermartlienshire beneath the Landeilo, It is clear 

 that if he claimed no well-defined base to the Llandeilo rocks in 

 this latter (their typical region), it was l);ciuse he saw them 

 passing into the overlying liala beds. There was, in the error 

 by which he placed /v^rc/ the Llandeilo, strata which were 

 really ii/>o;y them, no ground whatever for afterwards including 

 in his Silurian system, as a downward continuation of the Llan- 

 deilo rocks (wliichare the basal portion of the Bala group), the 

 whole Festiniog group of Sedgwick ; whose infra-position to the 

 Bala had been shown by the latter long before it was known to 

 be fossiliferous. 



It was, however, claimed by Murchison that no line of separa- 

 tion can be drawn between these two groups. The results of 

 Ramsay and of Salter, as set forth in the address of the former 

 before the Geological Society in 1S63, and more fully in the 

 Memoirs of the Geological Survey (vol. iii. part 2) published 

 in 1866, with a preface by himself, as the director of the Survey, 

 are completely ignored by Murchison. The reader familiar with 

 these results, of which we have given a summary, finds with sur- 

 prise that in the last edition of "Siluria,'' that of 1S67, they are 

 noticed in part, but only to be repudiated. In the five pages of 

 text which are there given to this great Middle Cambrian divi- 

 sion, we are told that the distinction between the Lower Trema- 

 doc and the Lingula flags " is difficult to be drawn," and that the 

 Upper Tremadoc slate passes into and forms the lower part of 

 the Llandeilo, "into which it graduates conformably " ("Siluria," 

 4th ed. p. 46). In each of these cases, on the contrary, accord- 

 ing to Ramsay, there is observed " a break very nearly complete 

 both in genera and sjiecies, and probable unconlorjnity ; " the 

 evidence of the pakvontological break being furnished by the care- 

 ful studies of Salter; while tliat of the stratigraphical break, as 

 we have seen, leaves no reason for doubt. (Mem. Geol. Sur. iii. 

 part 2, pp. 2, 161, 234). The student of " Siluria" soon learns 

 that in all cases where Murchisoji's pretensions were concerned, 

 the Ijook is only calculated to mislead. 



The reader of this history will now be able to understand why, 

 notwithstanding the support given by Barrande, by the Geological 

 Survey of Great Britain, and by most American geologists, to 

 the Silurian nomenclature of Murchison, it is rejected, so far as 

 tlie Lingula-ftags and the Tremadoc slates are concerned, by 

 Lyell, Phillips, Davidson, Harkness, and Hicks in England, and 

 by Linnarsson in Sweden. These authorities have, however, 

 admitted the name of Lower Silurian for the Bala group or 

 Upper Cambrian of Sedgwick ; a concession which can hardly 



be defended, but which apparently found its way into use at a 

 time when the yet unravelled perplexities of the Welsh rocks 

 led Sedgwick himself to propose, for a time, the name of Cambro- 

 Silurian for the Bala group. This want of agreement among 

 gcnloj^ists as to the nomenclature of the lower palxozoic rocks, 

 causss no Utile confusion to the learner. We have seen that 

 Henry Darwin Rogers followed Sedgwick in giving the name of 

 Cambrian to the whole palaeozoic series up to the base of the May 

 Hdl sand-tone ; and the same view is adopted by Woodward in 

 his Manual of the Mollusca. The student of this excellent book 

 will find that in the tables giving the geological range of the 

 Mollusca, on pages 124, 125, and 127, the name of Cambrian is 

 used in Sedgwick's sense, as including all the fossiliferous strata 

 beneath the May Hill sandstone. On page 123 it is however 

 expl.iined that Lower .Silurian is a synonym for Cambrian, and it 

 is so used in the body of the work. 



The distribution of the Lower and Middle Cambrian rocks in 

 Great Britain may now be noticed. The former, or Bangor group, 

 to which Murchison and the Geological Survey restrict the name 

 of Cambrian, and which they sometimes call the Longmynd, 

 bottom or basement rocks, occupy two adjacent areas in Caernar- 

 von and Merionethshire ; the one near Bangor, including Llan- 

 beris, to the north-east, and the other, induding Harlech and 

 Barmouth, to the south-east of Snowdon ; this mountain lying in 

 a synclinal between them, and rising 3,571 feet above the sea. 

 The great mass of grits or sandstones appears to be at the summit 

 of the group, but in the lower part the blue roofing slates of Llan- 

 beris are inteistratified in a series of green and purple slates, 

 grits, and conglomerates (some of the Welsh roofing-slates are, 

 however, supposed to belong to the Llandeilo). (Mem. Geol. 

 .Survey iii. part 2, pages 54, 25S). The Harlech rocks in this 

 north-western region are conformably overlaid by the Menevian, 

 followed by the true Lingula-flags, or Olenus beds, of the Middle 

 Cambrian. Upon these repose the Tremadoc slates, which are 

 not known in the other parts of Wales. The third area of Lower 

 Cambrian rocks is that already described at St. David's in Pem- 

 brokeshire, about too miles to the south-west ; and the fourth, 

 that of the Longmynd hills, about sixty miles to the south-east 

 of .Snowdon. The rocks of the Longmynd, like those of the 

 other Lower Cambrian are.as mentioned, consist principally of 

 green and purple sandstones with conglomerates, shales, and some 

 clay-slates. They occasionally hold Hakes of anthracite, and 

 small portions of mineral pitch exude from them in some locali- 

 ties. The only evidence of animal life yet found in the rocks of 

 the Longmynd is furnished by worm-ljurrows, the obscure re- 

 mains of a crustacean (the Pjl,eopy^c Kiviisayi) and a form like 

 Ilistioiit-nita. This latter organic relic, with worm-burrows, and 

 the fossils named OlJhamia, is found on the coast of Ireland 

 opposite Caernarvonshire, in the rocks of Bray Head ; which 

 resemble lithologicaUy the Harlech beds, and are regarded as 

 their equivalents. 



Still another area of the older rocks is that of the Malvern 

 hills, on the western flanks of which, as .already mentioned, the 

 Lingula flags are represented by about 500 feet of black shales 

 with 0/iinis, underlain by 600 feet of greenish sandstones con- 

 taining traces of fucoids, with Serpulites and an Ol>o!ellii. It is 

 not improbable, as suggested by Barrande and by Murchison, 

 that these 1,100 feet of strata represent, in this region, the great 

 miss of the Lingula flags, and, we may add, perhaps the whole 

 series of Lower Cambrian strata, which in Caernarvonshire and 

 Pembrokeshire underlie them ; since these sandstones of Mal- 

 vern, like those of St. David's, rest upon crj'stalline schists, and 

 are in part made up of their ruins. 



These crystalline schists of Malvern, which are described by 

 Phillips as the oldest rocks in England, and by .Mr. Hull are 

 conjectured to be Laurentian, seem, from the descriptions of their 

 lithological characters, to resemble those of Caernarvon and 

 Anglesea, with which they are, by Murchison, regarded as identi- 

 cal. The crystalline schists of these latter localities are by Sedgwick 

 described as hypozoic strata, below the base of the Cambrian. 

 Murchison, however, in the first edition of his " Sduria," adopted 

 the suggestion of De la Beche that they themselves were altered 

 Cambrian strata. In fact, they directly underlie the Llandeilo 

 rocks, and were apparently conceived by Murchison to represent 

 the downward continuation of these, upoi which he had insisted. 

 This opinion is supported by ingenious arguments on the part 

 of Rams.ay (Mem. Geol. Survey, iii. part 2, passim). I am 

 however disposed to regard them, with Sedgwick and Phillips, 

 as of pre-Cambrian age, and to compare them with the Huronian 

 series of North America, wdiich occupies a similar geological 



