804 THE WONDERS OF GEOLOGY. Lect. VIII. 



rocks have been exposed to a still higher temperature, the 

 transmutation is more complete; as for instance, when 

 granite has been erupted in a state of fusion among schis- 

 tose strata. The metalliferous veins so abundant in the 

 slate rocks are either intrusions of mineral matter into 

 pre-existing fissures, or sublimations of metallic substances 

 into cavities formed in the rock itself during its refrigeration, 

 and into which the metal was introduced by segregation. 



In fine, the Silurian and Cumbrian systems afford proofs 

 of marine depositions going on through immense periods 

 of time in seas inhabited by fishes, crustaceans, crinoi- 

 deans, corals, and mollusca, belonging to numerous genera 

 and species. For although organic remains prevail only in 

 the uppermost or newest system, yet as we have decided 

 proof that the lowermost has been subjected to intense 

 heat, and that even the lines of stratification are in a 

 great measure melted away, it is reasonable to conclude, 

 that the absence of fossils is attributable to the obliteration 

 of the remains of the animals which lived and died in the 

 waters that deposited these schistose rocks. We must, 

 however, remember, that the relics which remain are of a 

 peculiar type, and altogether different from those of the 

 newer secondary formations. 



With the lowest fossiliferous beds of the Silurian and 

 Cumbrian systems, we lose all positive evidence of the 

 presence of organic beings on the surface of the earth ; 

 but it would be rash in the extreme therefore, to assume, that 

 these most ancient fossils are the relics of the earliest living 

 things that tenanted our planet. Well has Mr. Lyell 

 remarked, that " it is too common a fallacy to fix the era 

 of the first creation of each tribe of plants or animals, and 

 even of animate beings in general, at the precise point where 

 our present retrospective knowledge happens to stop."* 

 * Travels in America. 



