460 Prof. T. Sterry Hunt — On Cambrian and Silurian. 



(including the Menevian) have in this region, according to Kamsay, 

 a thickness of about 6000 feet. Above these, near Tremadoc and 

 Ffestiniog, lie the Tremadoc slates, which are here overlaid, in 

 apparent conformity, by the Lower Llandeilo beds. At a distance of 

 eleven miles to the north-west, however, the Tremadoc slates dis- 

 appear, and the Lingula-flags are represented by only 2000 feet of 

 strata ; while in parts of Caernarvonshire, and in Anglesea, the whole 

 of the Lingula-flags and moreover the Lower Cambrian rocks, are 

 wanting, and the Llandeilo beds rest directly upon the ancient 

 crystalline schists. In Scotland and in Ireland, moreover, the 

 Lingula-flags are wholly absent, and the Llandeilo rocks there 

 repose unconformably upon grits regarded as of Lower Cambrian 

 age. Thus, without counting the Tremadoc slates, which are a local 

 formation, unknown out of Merionethshire, we have (including the 

 Bangor group and Lingula-flags), beneath the Llandeilo, over 9000 

 feet of fossiliferous strata, which disappear entirely in the distance 

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

 clusion of Eamsay 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 palseontological break. The 

 fauna of the Tremadoc slates is, according to Salter, almost entirely 

 distinct from that of the Lingula-flags, and not less distinct from 

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

 lents of the Skiddaw slates of Cumberland). Hence, says Eamsay, 

 it is evident " that in these strata we have three perfectly distinct 

 zones of organic remains, and therefore, in common terms, three 

 distinct formations." The palteontological evidence is thus in com- 

 plete accordance with that furnished by stratigraphy. We cannot 

 leave this topic without citing the conclusion of Eamsay 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 formations." In connexion 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, d priori, be expected to occur most frequently in regions where 

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

 lation of such masses implies great movements of subsidence, which, 

 in their nature, are limited, and are accompanied by elevations in 

 adjacent areas, from which may result, over these areas, either inter- 

 ruptions 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 different in a i-egion 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. 



