ELEMENTARY GEOLOGY. 25 



<; The bed3 above described are sometimes called Cambrian, but, as already ex- 

 plained, it is difficult to separate them with propriety from the Silurian series'.'' 



In the Journal of the Geological Society of Dublin, which is printed in 

 the present number of the Natural History Review, is a paper by J. R. 

 Kinahan, Esq., M.B., announcing the addition of a fresh and interesting 

 group of animals to the solitary Oldhamias of Bray Head ; we take this 

 opportunity of directing our readers' attention to it, as we believe it to be a 

 valuable addition to the scanty knowledge we possess of the Organisms of 

 the Cambrian Epoch. 



An interesting question is alluded to in the closing paragraph of section 

 (603), already quoted ; viz., where are the original rocks whose wear and 

 tear supplied the materials for the vast thicknesses of beds deposited during 

 the early (Silurian Epoch ? 



In connexion with this inquiry, we subjoin the following passage : — 



" 609. Commencing with the crystalline and altered slates of Cumberland, 

 North Wales, Scotland, and Ireland, it may be observed that the proportion of 

 argillaceous matter and quartz is much greater, and the mixture with calcareous 

 rocks less, than in strata of more recent date. Although of a thickness amounting 

 to many thousand feet, the sedimentary deposits forming the base of the Silurian 

 system are, almost without exception, composed of clay, pebbles, or boulders, and 

 siliceous sand ; while the frequent presence of mica seems to indicate a preponder- 

 ance of granitic rocks amongst those to whose degradation and disintegration this 

 long series of crystalline slates, mica-schists, micaceous sandstones, conglomerates, 

 imperfect and highly altered limestones, and quartz rock must be owing. 



" The unvaried character of these beds over large tracts of country, and the general 

 resemblance among the oldest sedimentary deposits, have been looked upon as a 

 strong argument in favour of the uniformity of the materials of which the original 

 framework of the solid surface of the globe was composed.'' 



The theory of " Contemporaneous Traps" now generally adopted would 

 suggest a source from which part, at least, of those vast deposits might 

 be derived ; but the greater portion of them must have had their origin 

 in the Mechanical degradation of pre-existing rocks. Where and what 

 are these rocks ? We feel disposed to agree with Prof. Ansted as to the 

 large quantity of Mica in the Silurian Slates, as such a quantity of Mica 

 must be accompanied with a corresponding proportion, more than one- 

 tenth, of alkalies (potash) ; and such an amount of alkalies is usually 

 found to exist in ordinary slates. It is, however, a subject not sufficiently 

 examined, and well worthy of attention. And it is worthy of considera- 

 tion whether the Mica in the more crystalline of the Azoic rocks be not 

 due to metamorphic action, and not to the degradation of granitic rocks. 

 Be this as it may, the fact is certain that in Great Britain and Ireland we 

 do not see the primitive rocks whose wear g and tear supplied the elements of 

 the slates of Wales, Cornwall, Cumberland, Down, Armagh, and Kildaf e. 



