182 Revieios — LyeWs Student's Elements of Geology. 



speculation which he had felt bound to go into more fully in his 

 former work. But new discoveries had to be noticed, new theories 

 considered, to bring the book up to the day, and it had to be entirely 

 recast. 



Several important changes bearing upon the classification of both 

 the older and newer rocks may be noticed ; sometimes involving a 

 change in the brackets by which we group together the strata accord- 

 ing to the evidence we have of the continuity or discordance of the 

 conditions which prevailed at successive periods ; and sometimes 

 a,rising from a more detailed subdivision of beds which have been 

 better worked out. For instance, the discovery of a rich fauna in 

 rocks of Cambrian age in North and South Wales, as well as on the 

 Continent, has called for a new nomenclature in that part of the series. 

 The reconsideration of the evidence for drawing the line between 

 Lower and Upper Silurian in the middle of the Llandovery rocks 

 has led our author to bracket those formations together, and draw 

 the strongest line provisionally^at the base of the Lower Llandovery, 

 (p. 452.) "I formerljr," says Sir Charles, " adopted the plan of those 

 who class them as Middle Silurian, but they are scarcely entitled to 

 this distinction, since, after about 1400 Silurian species have been 

 compared, the number peculiar to the group in question only gives 

 them an importance equal to such minor subdivisions as the Ludlow 

 or Bala groups. I therefore prefer to regard them as the base of the 

 Upper Silurian, to which group they are linked by more than twice 

 as many species as to the Lower Silurian. By this arrangement the 

 line of demarcation between the two great divisions, though con- 

 fessedly arbitrary, is less so than by any other." (p. 452.) It does 

 not matter so much whether they are grouped together as a Middle 

 Silurian or as a distinct division at the base of the Upper Silurian, 

 for our divisions are well known to be not of equal value ; but it is 

 very important to place these rocks together -in one group, instead 

 of making the Lower Llandovery belong to one great division of the 

 series, and the Upper Llandovery to another. 



After a re-examination in the field of the rocks of Devon, our 

 author speaks of them still as " the marine type of the British strata 

 intermediate between the Carboniferous and Silurian." (p. 431.) As 

 long as we cannot clearly correlate either newer or older Devonian 

 beds with rocks also known to be " intermediate between the Car- 

 boniferous and Silurian" on the other side of the Bristol Channel, we 

 may well be content to wait for further evidence before we come to 

 a definite conclusion as to whether or not we have in Devonshire the 

 exact equivalent of the great mass of Old Eed which succeeds the 

 Tilestones conformably in South Wales ; or whether we have beds 

 homotaxially related to the Lower Carboniferous with its Lower 

 Limestone shale in one area, and rough conglomerate in another. 

 Possibly these beds, being more closely allied to the Carboniferous 

 than to the true Old Eed, were deposited in the Devonian area while 

 a wave of depression was travelling from south to north, thus 

 making the deposits of the southern district of earlier date. At any 

 rate, the interval between the Silurian and Carboniferous is so 

 enormous that we Should have plenty of room for aU the deposits, 



