304 THE CANADIAN NATURALIST. [Vol. vi. 



more than 30,000 feet of sediments,^ which in Wales are their 

 paleontological 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 zoolo- 

 gical epochs whose place in the series is marked by no strati- 

 graphical break. In such comparatively stable regions, move- 

 ments of the surface sufficient to cause the exclusion, or the dis- 

 appearance by removal, of the small thickness of strata corres- 

 ponding to an epoch, may take place without any conspicuous 

 marks of stratigraphical discordance. 



The attempt to establish geological divisions or horizons upon 

 stratigraphical or paleontological breaks must always prove falla- 

 cious. From the nature of things, these, whether due to non-de 

 position or to subsequent removal of deposits, must be local ; and 

 we can say, confidently, that there exists no break in life or in sedi- 

 mentation which is not somewhere filled up and represented by 

 a continuous and conformable succession. While we may define 

 one period as characterized by the presence of a certain fauna, 

 which, in a succeeding epoch, is replaced by a difierent 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 stratified rocks into systems is therefore unphi- 

 losophical, if we assign any definite or precise boundaries or limi- 

 tations 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 systems natures. [Philos. Mag. 

 IV. viii, 359] 



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

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

 Bohemia, were marked by the first fauna ; although he, at the 



* The Longm5-nd rocks in Shropshire are alone estimated at 20,000 

 feet ; hut their supposed equivalents, the Harlech rocks of Pemhroke- 

 shire, have a measured thickness of 3,300, while the Llanberris and 

 Harlech rocks together, 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 m the Berwyns 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. Surrey, 

 III, part 2, pages 72, 222, and Siluria, 4th ed. 185.] 



