Notices respecting Neiu Books. 307 



Llandovery, which are unconformable to the lower, are placed in 

 the Upper Silurian. Professor Ramsay, indeed, states that " a great 

 physical break takes place at this- point, all the lower members of 

 the Silurian series having been disturbed and planed across by de- 

 nudation before the upper Llandovery beds were formed." The 

 Tarannon shales are sometimes to all appearance conformable to the 

 upper Llandovery, but sometimes distinctly overlap them ; but the 

 succeeding Wenlock and Ludlow beds are conformable to one an- 

 other and to the Tarannon shales beneath. In fact it appears that 

 there is an Upper Silurian series of conformable formations underlain 

 by three formations unconformable to one another, namely the Ta- 

 rannon shale, and the upper and lower Llandovery ; and these again 

 are underlain by a Lower Silurian series of conformable deposits. 

 Professor Ramsay's interpretation of these facts seems very probable — 

 namely, that the strata which "lie between the top of the Caradoc 

 or Bala beds and the base of the Wenlock shale .... were formed 

 during a period of frequent oscillation of the relative level of the 



land to the sea Three fragments of this episode have alone 



been preserved ; and while the oldest, that of the lower Llandovery 

 beds, is somewhat closely connected with the Lower Silurian period, 

 the remaining two are more nearly related to the Upper Silurian age." 



Though few geologists will be inclined to oppose this view, and 

 to give preference to that of a Middle Silurian epoch, there is another 

 question which we think will meet with a different reception. We 

 refer to the propriety of continuing to include the Lingula-flags in 

 the Lower Silurian series. On this question, indeed, Professor 

 Ramsay seems to accept a conclusion diametrically opposed to his 

 own reading of the evidence, and consequently, we imagine, to 

 his own convictions. We find him chronicling the unconformity of 

 the overlying strata to the Lingula-flags, and the conformity of the 

 latter to the Cambrians. He mentions the absence of graptolites 

 from the Lingula-flags, and their occurrence in the whole of the re- 

 maining members of the Silurian system. He also quotes Mr. Sal- 

 ter's determinations to show the almost entire distinctness of the 

 Lingula-flag, Tremadoc, and Llandeilo faunas ; and he even goes so 

 far as to say of the Cambrian deposits and the Lingula-flags, that, 

 " except for the convenience of a great lithological distinction, they 

 scarcely require separation by line and colour." All these admis- 

 sions make us wonder why it should be so vehemently insisted 

 upon that the Lingula-flags are Lower Silurian, not Cambrian. The 

 only argument in its favour appears to be the " convenience " of the 

 error. And thus our arrival at a natural classification must be post- 

 poned until " convenience " holds a lower rank than truth. 



In the body of the memoir Professor Ramsay describes the Cam- 

 brian and Silurian rocks of North Wales as they occur in several 

 typical areas ; but his method is neither quite geographical nor quite 

 stratigraphical. He has, however, succeeded in bringing out the 

 chief features of the structure of the country, by avoiding irrelevant 

 matters, and by giving a summary of the facts advanced, and infer- 

 ences drawn, in discussing each particular area or formation. 



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