376 CAMBRIAN AND SILUltlAN IX EUROPE. [XV. 



The fauna of the Tremadoc slates is, according to Salter, al- 

 most entirely distinct from that of the Lingula flags, and not 

 less distinct from that of the so-called Lower Llandeilo or 

 Arenig rocks (the equivalents of the Skiddaw slates of Cum- 

 berland). Hence, says Ramsay, it is evident "that in these 

 strata we have three perfectly distinct zones of organic re- 

 mains, and therefore, in common terms, three distinct forma- 

 tions." The pala3ontological evidence is thus in complete 

 accordance with that furnished by stratigraphy. We cannot 

 leave this topic without citing the conclusion of Ramsay that 

 "each of these two breaks necessarily implies a lost epoch, 

 stratigraphically quite unrepresented in our area; the life of 

 which is only feebly represented in some cases by the fossils 

 common to the underlying and overlying formation." In 

 connection with this remark, which we conceive to embody 

 a truth of wide application, it may be said that stratigraphical 

 breaks and discordances in a geological series may, a priori, 

 be expected to occur most frequently in regions where this 

 series is represented by a large thickness of strata. The accu- 

 mulation of such masses implies great movements of subsi- 

 dence, which, in their nature, are limited, and are accompa- 

 nied by elevations in adjacent areas, from which may result, 

 over these areas, either interruptions in the process of sedi- 

 mentation, or the removal, by sub-aerial or sub-marine denuda- 

 tion, of the sediments already formed. The conditions of 

 succession and distribution, it may be conceived, would be 

 very different in a region where the period corresponding to 

 this same geological series was marked by comparatively small 

 accumulations of sediment upon an ocean-floor subjected to no 

 jjvat movements. 



This contrast is strikingly seen between the conformable 

 of less than 2,000 feet of strata, which in Scandinavia 

 are Characterized by the first three palaeozoic faunas (Cambrian 

 ami Silurian), and the repeatedly broken and discordant suc- 

 cession of more than 30,000 feet of sediments,* wlnYh in 



The Longmyml rocks in Shropshire are alone estimated at 20,000 feet ; 

 but their supposed equivalents, the Harlech rocks of Pembrokeshire, have a 



