XV.] CAMBRIAN AND SILURIAN IN EUROPE. 377 



Wales are their palaeontological equivalents. It must, however, 

 be considered that in regions of small accumulation where, as 

 in Scandinavia, the formations are thin, there may be lost or 

 unrepresented zoological periods whose place in the series is 

 marked by no stratigraphical break. In such comparatively 

 stable regions, movements of the surface sufficient to cause the 

 exclusion, or the disappearance by removal, of the small thick- 

 ness of strata corresponding to a zoological period, may take 

 place without any conspicuous marks of stratigraphical dis- 

 cordance. 



The attempt to establish geological divisions or horizons 

 upon stratigraphical or palseontological breaks must always 

 prove fallacious. From the nature of things, these, whether 

 due to non-deposition or to subsequent removal of deposits, 

 must be local ; and we can say, confidently, that there exists 

 no break in life or in sedimentation which is not somewhere 

 filled up and represented by a continuous and conformable suc- 

 cession. While we may define one period as characterized by 

 the presence of a certain fauna, which, in a succeeding age, is 

 replaced by a different one, there will always be found, in some 

 part of their geographical distribution, a region where the two 

 faunas commingle, and where the gradual disappearance of the 

 old before the new may be studied. The division of our strati- 

 fied rocks into systems is therefore unphilosophical, if we 

 assign any definite or precise boundaries or limitations to these. 

 It was long since said by Sedgwick with regard to the whole 

 succession of life through geologic time, that all belongs to 

 one great systema naturae. (Philos. Mag. (4), VIII. 359.) 



We have already noticed that Barrande, as early as 1852, 

 gave the name of Primordial Silurian to the rocks which, in 



measured thickness of 3,300, while the Llanberris and Harlech rocks to- 

 gether, in North Wales, equal from 4,000 to 7,000 feet, and the Lingula flags 

 and Tremadoc slates, united, about 7,000 feet. The Bala group in the Ber- 

 wyns exceeds 12,000 feet, and the proper Silurian, from the base of the 

 Upper Llandovery or May Hill sandstone, attains from 5,000 to 6,000 feet ; 

 so that the aggregate of 30,000 feet may be considered below the truth. 

 (Mem. Geol. Survey, III., Part II. pages 72, 222 ; and Siluria, 4th ed. 185.) 

 [The aggregate thickness since assigned to these rocks by Hicks is about 

 33,000 feet.] 



