SUPERPOSITION NOT PARENTAL RELATION. 209 



place at least not earlier than the times of the Cambrian 

 system. 



There is another consideration, of at least equal, if not 

 greater weight. A general correspondence is found to ob- 

 tain in widely-separated localities, in the organic contents of 

 that lowest band of the Lower Silurian or Cambrian system 

 in Avhich fossils have been detected. In Kussia, in Sweden, 

 in Norway, in the Lake district of England, and in the 

 United States, there are certain rocks which occupy relative- 

 ly the same place, and enclose what may be described gene- 

 rally as the same remains. They occur in Scandinavia as 

 that " fucoidal band" of Sir Roderick Murchison which forms 

 the base of the vast Paleozoic basin of the Baltic ; they exist 

 in Cumberland and Westmoreland as the Skiddaw slates of 

 Professor Sedgwick, and bear also their fucoidal impressions, 

 blent with graptolites ; they are present in North America 

 as those Potsdam sandstones of the States' geologists in which 

 fucoids so abound, mixed with a minute lingula, that they 

 impart to some portions of the strata a carboniferous charac- 

 ter. But with these deep-lying beds in all these several lo- 

 calities, thousands of miles apart, fossils cease. And why 

 cease with them 1 In one locality the ancient ocean may 

 have been of such a depth in the period immediately joreviows, 

 and represented, in consequence, by the strata immediately 

 beneath, that no animal could have lived at its bottom, — 

 though I do not well see why the remains of those animals 

 who, like the shark and pilot-fish, are frequently seen swim- 

 ming over the profoundest depths, might not, did such exist 

 at the time, be, notwithstanding, found at its bottom ; or in 

 another locality every trace of organization in the nether 

 rocks may have been obliterated, at some posterior period, 

 by fire. ' But it is difficult to imagine that that uniform cessa- 

 tion of organized life at one point which seems to have con- 

 ducted Sir Roderick Murchison and Professor Sedgwick to 



o 



