SLATE SYSTEMS OF ROCKS. 



38. This system presents us with depots of the oldest com- 

 bustible materials known ; and we find in it ferns, ca'lamites, divers 

 species of plants, differing but little from the plants found in the 



coal formation which 

 immediately follows. 

 We here find also a 

 great many pol'yps 

 more or less analogous 

 to the CaruophyUia 

 (Jig. 21) ; Jimplexus 

 (Jig. 22), by some re- 

 garded as polyps and 

 by others as chamber- 

 ed shells, which are 

 found nowhere beside. 



Fig. 25. Megalodon cuculla'tus. - -, ,. - 



so nearly resembling 



certain productus, appears to be characteristic of the Devonian 

 locks ; and perhaps also the Clymenia Hnearis (Jig. 24), a cham- 

 fered shell with aventral siphon. Certain peculiar bivalves are 

 a] so found (fig. 25); some brachiopods, and among others the 

 7'erebra'tnla porrecta (Jig. 26). 



39. Slates, so extensively used for roofs, are furnished from this 



group of ancient rocks ; 

 and on many we find im- 

 pressions of trilobites. The 

 upper part of the transition 

 strata often contains car- 

 boniferous materials, some- 

 times disseminated among 

 the schists, and at others 

 constituting more or less ex- 

 tensive masses, which are 



generally composed of anthracite, though sometimes of bituminous 



coal. 



40. Thefce three systems of rocks, namely the Cambrian, Silu- 

 rian and Devonian, which are not easily distinguished from each 

 other, are found in most countries of Europe, where their 

 assemblage constitutes the greater part of what is named the 

 transition or paleozoic formation. They abound in Brittany : 

 there the anthraciti'ferous mass forms a stripe along the Loire, ex- 

 tending from Maine to Morbihan, as well as other depots in Sarthe 

 and Mayenne. These rocks are found through the whole chain 



38. What fossils are found in the Devonian System ? 



39. What useful material is found in the Devonian System ? 



40. What systems of rocks constitute the palaeozoic formation ? Where 

 is this formation met with ? 



Fig. 26.-4erebra'tula porrecta. 



