Ch. XXVL] silukian stkata. 549 



nation hitherto advanced save that afforded by the theory of progres- 

 sive development. Nothing is known of the insects, land-shells, or 

 other terrestrial animals which coexisted with this Devonian flora, but 

 we need not despair of future discoveries in this direction when we 

 remember that slow as has been our progress, we have at length be- 

 gun to learn something respecting the terrestrial fauna of the Coal 

 period. 



Allusion has already been made to freshwater shells and to Lepi- 

 dodendra and ferns (see figs. 585 and 586, p. 524) fomid in Ireland 

 associated with Devonian genera of fish. 



CHAPTER XXVII. 



SILURIAN AND CAMBRIAN GROUPS. 



Silurian strata formerly called Transition — Term " Grauwacke " — Subdivisions of 

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

 known remains of fossil fish — Wenlock formation, corals, cystideans, trilobites — 

 Middle Silurian or Llandovery Beds — Lower Silurian rocks — Caradoc and Bala 

 Beds — Upper and Lower Llandeilo formations — Cystideae — Trilobites — Grapto- 

 lites — Vast thickness of Lower Silurian strata, sedimentary and volcanic, in 

 Wales — Foreign Silurian equivalents in Europe — Silurian strata of the United 

 States — Amount of specific agreement of fossils with those of Europe — Canadian 

 equivalents — Whether Silurian strata of deep-sea origin — Cambrian rocks — 

 Classification and nomenclature — Barrande's primordial fauna — Upper Cam- 

 brian of Wales — Tremadoc slates — Lingula flags — Lower Cambrian — Long- 

 mynd group — Oldest organic remains known in Europe — Foreign equivalents of 

 the Cambrian group — Primordial zone of Bohemia — Characteristic trilobites — 

 Metamorphosis of trilobites — Alum schists of Sweden and Norway — Potsdam 

 sandstone of United States and Canada — Footprints near Montreal — Quebec 

 strata and Huronian rocks — Minnesota trilobites — Rocks older than the Cam- 

 brian — Laurentian group, Upper and Lower — Oldest known fossil, JEozoon Cana- 

 dense — No remains of vertebrate animals known in strata below the Upper 

 Silurian — Progressive discovery of vertebrata in older rocks — Theoretical infer- 

 ences from the rarity or absence of vertebrata in the most ancient fossiliferous 

 formations. 



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 rea- 

 sons explained in Chapter VIII., p. 89. Geologists were also 

 in the habit of applying 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 to- 



