70 REPORT-^1875. 



of these pass up, and a few others come in, a little higher in the series, all in soft 

 calcareous sandy shales, with here and there limestone nodules and bands of lime- 

 stone ; and that when a decided change in the strata takes place, indicating- in some 

 places, by more siliceous and gritty beds, comparatively shallow water deposits, 

 and in others, by excessively hard hne-graiued limestones, a deeper sea, a decided 

 change in the Graptolite fauna occm-s — the gi'itty beds containing in myriads a 

 single new species, Monograpius leintumrdensis; and the indurated limestone alone 

 containing the few species of the Cladophora, belonging to the genus Ptilo;/rapius, 

 which have yet been detected. Monograptus tolomts (a form passing up from the 

 lowest Llandovery beds) appeared to be the only species which sui'vived these 

 physical clianges, it having alone been seen in the softer beds high in the Lower 

 Ludlow, and passing up from tliese into the harder calcareous shales, which in some 

 places immediately underlie the Aymestry Limestone, when this is at the summit of 

 the Lower Ludlow, and again passing up into this limestone bed, in which it seems 

 finally to disappear. 



The author considered this Aymestry Limestone to form a portion only of the 

 Lower Ludlow rocks, not being constantly present, and sometimes having beds of 

 Lower Ludlow shales of considerable thiclmess bet%veen its layers ; and concluded by 

 showing the dependence of the fossil fauna and flora of these rocks on the physical 

 conditions of the Lower Ludlow seas, the fossils frequently' being only locally dis- 

 tributed, and varying slightly in their horizons according to the nature of the sedi- 

 ment deposited, the Graptolites especially being influenced by these changes, to 

 which their final extinction, or at least their dispersion from the area under con- 

 sideration, was considered to have been most probably due. 



To the list previously given a single species only, Munograptiis Mvemcri, Bai-- 

 rande, occurring in the lowest beds of the Lowei* Ludlow, is added by these recent 

 researches. 



Notes on the Classification of the Sedimentary Itocl-s. Bif T. M'^K. Hughes, 

 M.A., F.S.A., F.G.S., Woodwardian Professor of Geology, C'amhridr/e. 



Prof. Hughes, in advocating a revision of the classification of the sedimentai-y 

 rock's, pointed out : — 



(1) That, although the accumulation of rock-material may have been going on 

 somewhere throughout the whole of the periods -with which geologists have to do, 

 still that deposition has been locally inteiTupted many times ; 



(2) That the whole evidence had to be considered in each case, as it was a matter 

 of every-day observation that trifling geographical changes might produce con- 

 siderable alteration not only in the character of the sediment, but also in the faima 

 and flora of a given area; and small local in-egularities, due for instance to volcanic 

 action, might produce phenomena which alone would be taken for an unconformity, 

 implying a long interniption of deposit ; 



(3) That denudation proves a lapse of time somewhat commensurate with the 

 deposition of a similar thickness of material to that denuded. 



Bearing these principles in mind, he observed that the great divisions should be 

 drawn where it can be sliown there was the greatest and longest interruj)tion in 

 the continuity of conditions, the minor subdivisions being founded upon more 

 rapidly varying circumstances, which often produce even greater diflference in litho- 

 logical character and fossil contents. 



He pointed out that our present classification was vei-j inconsistent — some of 

 the breaks within the Primary, for example, being far more important than that 

 between the Primary and Secondary rocks themselves. 



He proposed the foUowing classification, read in ascending order : — 



1st Z^poch. Laurentian. 



2nd „ Gap. 



iird ,, .Labrador Scries, y Pre- Cambrian. 



4th „ Gaj}. 



5th „ Huronian ? 



The Huronian he felt to be not quite well defined, but thought it probable that 



