§ 7. CHRONOLOGICAL SYNOPSIS. 205 



quartz pebbles, cemented together by red sand and marl, and have 

 evidently resulted from the destruction of ancient slate rocks j they are 

 in many instances mixed up with volcanic ashes and scoriae. Beautiful 

 coralline marbles occur in South Devonshire, and in some localities 

 fishes of a very remarkable conformation, belonging to genera unknown 

 in any other system. 



9. the Silurian formation. — Marine limestones, 

 sandstones, shales, and calcareous flagstones, abounding in 

 shells, cephalopoda, corals, and crustaceans, (especially of 

 the extinct family of Trilobites,) of peculiar types, constitute 

 this important and extensive system of marine deposits. 

 A few traces of fuci are the only known vegetable remains. 



Observations. — This system (which was first established by Sir K. 

 Murchison) is entirely of marine origin, and many beds are 

 aggregations of corals, shells, &c. cemented together by calcareous 

 matter. The organic remains are numerous, and afford characters 

 by which the system is conveniently subdivided into two groups ; but 

 only one general organic type prevails throughout. The lowermost 

 beds contain but very few fossils, and insensibly pass into sedimentary 

 deposits, in which no traces of organic existence have yet been 

 discovered. 



10. THE CUMBRIAN ( ScllistOSC ) FORMATION. A largely 



developed series of slate rocks, and conglomerates, several 

 thousand yards in thickness ; the uppermost beds contain 

 a few shells and corals, which are not distinguishable 

 from Lower Silurian species. 



Observations. — This system includes all the sedimentary deposits 

 between the lowermost of the Silurian and the uppermost of the 

 ancient metamorphic rocks. It has been named by Professor Sedgwick 

 Cambrian, and Cumbrian, from the large development of the strata in 

 Wales and Cumberland. But as no assemblage of organic remains 

 specifically distinct from the fossils of the Silurian has been detected, 

 a zoological line of separation cannot at present be drawn between the 

 lower Silurian and the Cambrian deposits. Until characteristic fossils 

 be met with, the group, therefore, as a distinct system, rests on the 

 geological position and lithological characters of the strata so largely 

 developed in Cumberland. Future researches will, in all probability, 

 lead to the discovery of peculiar organisms, more ancient than the 

 Silurian types. 



