§ 27. REVIEW OF THE SILURIAN AND CUMBRIAN SYSTEMS. 803 



kingdom ; not a vestige of any terrestrial animal or plant 

 has been discovered. These organic remains belong for 

 the most part to peculiar types, some of which extend 

 into the upper palaeozoic formations, but none occur in the 

 secondary deposits. 



In the Cumbrian system, we have a vast argillaceous 

 formation, with numerous conglomerates ; and from the 

 structure of the entire series, it would appear that after 

 the deposition of the strata by water, the whole had been 

 exposed to the long-continued influence of heat, by which 

 the original sedimentary character was either greatly 

 modified, or entirely obliterated. As a few fossils occur 

 in the upper part of the system, it will probably hereafter 

 be found convenient to separate the Silurian from the 

 Cumbrian at a lower level than the original base line, and 

 thus include the fossiliferous strata in the former grand 

 division. In accordance with the slaty structure, is the 

 prevalence of igneous rocks throughout the Cumbrian 

 series ; for granite, porphyry, serpentine, and trap, occur 

 not only in veins and dikes, but are also intercalated with 

 the strata, as if the melted matter had been poured over 

 argillaceous sediments at the bottom of the sea, and was 

 covered by succeeding deposits. 



When dikes of basalt or trap traverse or intersect the 

 limestones or shales, we find them indurated, and some- 

 times altogether changed in their lithological characters. 

 In the slates, the lines of stratification are more or less 

 manifest, and the rocks have a cleavage, that is, a tendency 

 to split in directions which bear no relation to the lines of 

 deposition, as I have already explained (ante, p. 798), 

 but have resulted from exposure to a high temperature, by 

 which the character and arrangement of the constituent 

 molecules of the rock have been altered (Lign. 182. 183); 

 for a tendency to a similar structure prevails when argil- 

 laceous beds are in contact with lava. Where slate 



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