No. 3.] HUNT — ON CAMBRIAN AND SILURIAN. 303 



the distance of a few miles. From a careful survey of all the facts, 

 the conclusion of Ramsay is irresistible, that there exists between 

 the Lingula-flags and the Llandeilo not merely one, but two 

 great stratigraphical breaks in the succession ; the one between 

 the Lingula-flags and the Lower Tremadoc slates, and the other 

 between the Upper Tremadoc slates and the Lower Llandeilo. 



This conclusion is confirmed by the fact that there exists at 

 each of these horizons a nearly complete paleontological break. The 

 fauna of the Tremadoc slates is, according to Salter, almost en- 

 tirely distinct from that of the Lingula-fl;igs, and not less distinct 

 from that of the so-called Lower Llandeilo or Arenig rocks, (the 

 equivalents of the Skiddaw slates of Cumberland). Hence, says 

 Ramsay, it is evident " that in these strata we have three per- 

 fectly distinct zones of organic remains, and therefore, in com- 

 mon terms, three distinct formations." The paleontological 

 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 forma- 

 tion." In connection with this remark, which we conceive to 

 embody a truth of wide application, it may be said that strati- 

 graphical breaks and discordances in a geological series, may, 

 d priori, be expected to occur most frequently in regions where 

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

 cumulation of such masses implies great movements of subsi- 

 dence, which, in their nature, are limited, and are accompanied 

 by elevations in adjacent areas, from which may result, over these 

 areas, either interruptions in the process of sedimentation, or the 

 removal, by sub-aerial or sub-marine denudation, of the sediments 

 already formed. The conditions of succession and distribution 

 it may be conceived, would be very difl'erent 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 great movements. 



This contrast is strikingly seen between the conformable series 

 of less than 2,000 feet of strata which in Scandinavia are char- 

 acterized by the first three paleozoic faunas (Cambrian and Si- 

 lurian) and the repeatedly broken and discordant succession of 



