264 



LYELL'S ELElMENTS OF GEOLOGY. 



Primary Fossiliferous Strata. 



CHAPTER XXII. 



PRIMARY FOSSILIFEROUS STRATA. 



Primary Fossiliferous or Transition Strata — Term "Grauwacke" — Silurian 

 Group — Upper Silurian and Fossils — Lower Silurian and Fossils — Trilobites — 

 Graptolites — Orthocerata — Occasional horizontality of Silurian Strata — Cambrian 

 Group — Endosiphonite. 



We have now arrived in the descending order at those more 

 ancient sedimentary rocks, which I have called the Primary 

 Fossiliferous (see p. 157.), and to which Werner first gave the 

 name of Transition, for reasons fully explained and discussed in 

 the 12th chapter. Many geologists have also applied to these 

 older strata the general name of " grauwacke," by which the 

 German miners designate a variety of quartzose sandstone, 

 which is usually an aggregate of small fragments of quartz, 

 flinty-slate (or Lydian stone), and clay-slate, cemented together 

 by argillaceous matter. But far too much importance has been 

 attached to this kind of rock, as if it were peculiar to a certain 

 epoch in the earth's history, whereas a similar sandstone or grit 

 is not only found sometimes in the Old Red, and in the millstone 

 grit of the Coal, and in certain cretaceous formations of the 

 Alps — but even in some tertiary deposits. . 



In England, the Old Red sandstone has been generally regard- 

 ed as the base of the secondary series ; but by some writers on 

 the Continent, the Old Red and Coal formations have been class- 

 ed as the upper members of the Transition series, a method 

 adopted by Dr. Buckland, in his late Bridgewater Treatise. This 

 classification, however, requires us to draw a strong line of de- 

 marcation between the Coal and the lower New Red sandstone 

 group, which, now that the fossils of these two groups are ascer- 

 tained to be very analogous, becomes a more arbitrary division 

 than that which separates the Old Red from the uppermost of 

 the primary fossiUferous strata. 



Professor Sedgwick and Mr. Murchison have lately proposed 

 to subdivide all the English sedimentary strata below the Old 

 Red sandstone into two leading groups, the upper of which may 

 be termed the Silurian, and the inferior the Cambrian system. 

 Mr, Murchison has applied the name of Silurian to the newer 

 group, because these rocks may be best studied in that part of 



