3j2 GEOLOGICAL EPOCH OF AN UPHEAVAL. 



Pebble Beds indicate Upheaval. When we find traces of a sudden 

 and complete change in the succession of aqueous deposits, so that the 

 quiet deposition of clay or limestone is interrupted by a tumultuous 

 aggregation of pebbles, we know that there has been some agitation 

 of the water. This may have happened either from a periodical 

 or accidental change in the drainage of the neighbouring land, or from 

 some extensive change in the relations of land and sea. The latter 

 interpretation may be adopted if these indications of agitation are 

 very extensive, arid if there be proof of local conglomerates following 

 upon local convulsions. 



Another indication of some distant convulsion affecting the 

 relations of land and sea seems to be afforded from the occurrence 

 of a bed of marine shells among fresh- water estuary deposits without 

 any local unconformity of stratification ; though the alternation of 

 fresh-water and marine conditions in the Lym Fiord in Jutland 

 shows that prevailing winds changing their direction or force may 

 account for some such phenomena. 



Method of Fixing the Age of an Upheaval. In geological in- 

 quiries concerning time, the answer is always expressed in terms of the 

 relative antiquity of stratified rocks ; and a convulsion is fixed in geolo- 

 gical time when it can be shown to have happened after the deposition 

 of one stratum, and before the deposition of another. If the strata 

 which thus limit the period of the convulsion be consecutive terms 

 of the series of deposits, the most precise result is obtained ; but if 

 these limiting strata be not consecutive, the age of the disturbance 

 is known only within a given range. 



An example of accurate determination of the geological era of a 

 convulsion is afforded in the North of England, where the newest of 

 the coal strata are found to be dislocated under the oldest red sand- 

 stones of the Permian system. 



Instances of less precise determinations are common enough : for 

 example, in the Mendip hills the dislocated mountain limestone is 

 covered by undisturbed oolite ; and, as far as this observation goes, 

 the convulsion may have happened during any part of the long period 

 occupied in producing the coal, red marl, and lias strata. In this 

 case, however, by tracing the line of the dislocation to other localities, 

 other strata are found to be so related to the limestone as to fix the 

 geological date of its disturbance within narrower limits. 



If the dislocated strata be not actually seen covered by others 

 which are undisturbed, another set of data must be employed. It 

 may happen that around the disturbed rocks some newer stratum 

 spreads in such a manner as to give sufficient reason to conclude that 

 it was deposited since the period of the convulsion. This is, in most 

 parts, all that can be observed with respect to the red marl around 

 the igneous rocks of Charnwood Forest, and there would be satisfactory 

 evidence that the slaty rocks of that district were upraised before 

 the period of the New Red Sandstone ; and, in fact, we have found 

 instances where the red marl does really cover with level beds the 



broken edges of slate. 





