SUCCESSION OF STRATA. 191 



of s!ate rocks, their detritus being cemented together by red sand, 

 or marl, into coarse conglomerates. 



Primary Fossiliferous Period. 



10. THE SILURIAN SYSTEM. This name is derived from silures, 

 the name of the ancient Britons who inhabited that part of Eng- 

 land where it is most developed, viz : the border counties of 

 England and Wales, and south Wales. It is a marine deposit of 

 vast extent and importance, containing a great abundance of or- 

 ganic remains- It is principally composed of marine limestone, 

 shales, sandstones, and calcareous flags, abounding in shells, 

 corals, trilobites, and crinoidea of peculiar types; but few vegeta-\ 

 ble remains are found below the old red sandstone. 



11. THE CAMBRIAN, OR GRAUWACKE SYSTEM. Grauwacke is 

 a coarse slaty rock, containing fragments of other rocks, some- 

 times passing into the common clay slate, and sometimes, when 

 the fragments are very numerous and small, into sandstones and 

 grits; it contains a few shells and corals, and occasionally im- 

 pressions of fuci; with this system all traces of organic remains 

 disappear. The fineness of grain, general aspect, and character 

 of these rocks, are well known from the universal employment 

 of slate for economic purposes. 



II. HYPOGENE OR METAMORPHIC ROCKS. 



Destitute of Organic Remains. 



STRATIFIED. 



12. THE MICA SCHIST. This formation is supposed from cer- 

 tain traces of stratification, to have been sedimentary in its origin, 

 but subsequently altered by the influence of heat. It consists of 

 mica slate, granite rock, crystaline limestone, or white marble, 

 and hornblende schist, exhibiting no traces of organic remains. 



13. THE GNEISS SYSTEM. Layers of gneiss, sienite, and quartz 

 rock, alternating with clay slate, and mica schist, but still exhib- 

 iting marks of former stratification. 



UN8TRATIFIEU. 



14. GRANITIC SYSTEM.- Consisting of porphyry, serpentine, 

 and trap rock in shapeless masses, and in dykes and veins. 



