Ch. XXYH.] SILURIAN" STRATA. 429 



CHAPTER XXVIL 



SILURIAN AND CAMBRIAN GROUPS. 



Silurian strata formerly called Transition — Term Grauwacke — Subdivisions of 

 Upper, Middle, and Lower Silurians — Ludlow formation and fossils — Ludlow 

 bone-bed, and oldest known remains of fossil fish — "Wenlock formation, corals, 

 cystideans, trilobites — Middle Silurian or Caradoc sandstone — Its unconforma- 

 bility — Pentameri and Tentaculites — Lower Silurian rocks — Llandeilo flags — 

 Cystidese — Trilobites — Graptolites — Vast thickness of Lower Silurian strata in 

 Wales — Foreign Silurian equivalents in Europe — Ungulite grit of Russia — 

 Silurian strata of the United States — Amount of specific agreement of fossils 

 with those of Europe — Canadian equivalents — Deep-sea origin of Silurian 

 strata — Fossiliferous rocks below the Llandeilo beds — Cambrian group — Lin- 

 gula flags of North "Wales — Lower Cambrian — Oldest known fossil remains — 

 " Primordial group" of Bohemia — Characteristic trilobites — Metamorphosis of 

 trilobites — Alum schists of Sweden and Norway — Potsdam sandstone of United 

 States and Canada — Footprints near Montreal — Trilobites on the Upper Mis- 

 sissippi — Supposed period of invertebrate animals — Upper Silurian bone-bed 

 — Absence of fish in Lower Silurian — Progressive discovery of vertebrata in 

 older rocks — Inference to be drawn from the greater success of British Pa- 

 leontologists — Doctrine of the non-existence of vertebrata in the older fossilif- 

 erous periods premature. 



We come next in the descending order to the most ancient of the 

 primary fossiliferous rocks, that series which comprises the greater part of 

 the strata formerly called " transition" by Werner, for reasons explained 

 in chap, viii., pp. 91 and 93. Geologists were also in the habit of ap- 

 plying to these older strata the general name of " grauwacke," by which 

 the German miners designate a particular variety of sandstone, usually an 

 aggregate of small fragments of quartz, flinty slate (or Lydian stone), and 

 clay-slate cemented together by argillaceous matter. Far too much im- 

 portance has been attached to this kind of rock, as if it belonged to a 

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

 grit is found in the Old Red, and in the Millstone Grit of the Coal, 

 and sometimes in certain Cretaceous and even Eocene formations in the 

 Alps. 



The name of Silurian was first proposed by Sir Roderick Murchison 

 for a series of fossiliferous strata lying below the Old Red Sandstone, and 

 occupying that part of Wales and some contiguous counties of England 

 which once constituted the kingdom of the Silures, a tribe of ancient 

 Britons. The following table will explain the various formations into 

 which this group of ancient strata may be subdivided. 



