A. J. Jukes- Browne — On Rock- Classification. 293 



Fig. 6. Supposed Cycadean (?) stem, Mt. Babbage (or Mt. Adams ?). Cast of 



portion of fossil wood (replaced by quartzite) bored by Teredo 

 (Carruthers,) (nat. size). 



,, 7, a, b. Supposed ends of permanent bases of leaves from stem (Fig. 6), con- 

 sidered by Mr. Carruthers to be casts of ends of Teredo-ixi^es. 



, , 7c. Profile of some of the supposed bases of leaves detached from stem, 



like Fig. 6. 



,, 8. Part of stem of Bucklandia Milleriana, Carr. {op. cit. tab. Iv. fig. 1, 



p. 687), original from the Coral Eag of Brora, Sutherlandshire 

 (half nat. size). 



II. — The Classification of Stratified Kocks. 

 By A. J. Jukes-Bkowne, B.A., F.G.S. 



MANY attempts to revive and improve the classification of our 

 rock-groups have been made from time to time, and the Inter- 

 national Geological Congress w^ill doubtless make a further effort in 

 the same direction. 



Systematists seem now generally to agree in allovping four ranks 

 to the divisions of the geological series : — 



1. Such divisions as Palasozoio and Neozoic, or Primary, Secondarj^ 



and Tertiary. 



2. The subordinate divisions which are generally termed sj'stems, 



such as Silurian, Cretaceous, etc. 



3. The subdivision of these systems into series usually called Upper, 



Middle and Lower, but sometimes by special names. 



4. The smaller local groups or stages into which the subdivisions 



naturally fall, such as Oxford Clay, Millstone Grit, etc. 



But when they come to rank the recognized groups under these 

 heads, and to decide upon the number of systems which should be 

 admitted into the geological scale, then there is much difference of 

 opinion. 



The latest attempt to improve our classification is by Dr. Blanford, 

 in the Geological Magazine for 1884, p. 318, and with his pre- 

 liminary remarks I think all geologists must cordially agree. He 

 points out that if our classification is to have anything like universal 

 afjplication, the first essential condition is that the major divisions 

 should be of nearly equal value ; that if systems are determined by the 

 thickness of strata or by the physical breaks which occur in any one 

 country, they cannot be of equal value : consequently the logical 

 conclusion is that systems must be established on a palseontological 

 basis, and on the evidence of change in the marine faunas. 



To concede that unconformities are geological accidents and of 

 little use in classification is a step in the right direction.^ If they 

 were always taken as the limits of systems, several different classi- 

 fications would be required for the British strata alone. A single 

 instance will suffice to prove this : — In Scotland, and probably in 

 Wales also, there is an upward passage from Silurian into Lower 

 Old Red Sandstone, then an unconformity and a passage from Upper 

 O.R.S. into Carboniferous ; here therefore there are only two systems 

 if the break is to be the limit ; but let us pass into Devonshire, and 

 1 Vide Lapworth, Geol. Mag. Deo. II. Vol. VI. p. 10. 



