RARITY OF POST-ARCHAEAN LIMESTONES. 85 



Iii the Canadian group, presumed to correspond with our 

 Upper Cambrians, we have evidence that a further increase of 

 limestones took place, while the Chazy limestone (considered 

 to be the youngest member of this group) and the Trenton lime- 

 stones of the Lower Silurian system afford ample evidence of an 

 abundant increase of the calcareous element. 



With reference to the large increase of limestones in the last- 

 named formations, the fact must be taken as showing that in 

 North America the increase took place earlier than in the Euro- 

 pean areas. 



Cconnected with this is the remarkable fact, already stated, 

 that the earliest Palaeozoic formations (those constituting the 

 two Cambrians) contain few fossils with ordinary calcareous 

 skeletons. Besides some others (fucoids) of no importance to the 

 question at issue, the Cambrians yield the remains of Protozoans, 

 Crustaceans, Coelenterates, Mollusks, and Echinoderms, occa- 

 sionally in abundance, and some of them (Paradoxides) of 

 gigantic proportions ; instead, however, of the skeleton of these 

 organisms being ordinarily thick and composed of lime, as is 

 general in certain of their classes respectively, they are (admit- 

 ting a few apparent exceptions) thin, and for the most part 

 horny, with comparatively a small quantity of phosphate of lime, 

 and a much smaller of the carbonate a circumstance which 

 may be taken as favouring the idea that the Cambrian stages of 

 organic development were not much beyond that of the larval 

 evolution of the invertebrates whose remains have been noticed*. 

 The absence of calcareous fossils, and the rarity of limestones 

 in the Cambrians, it may therefore be assumed, are correlative 

 phenomena. It would seem that the seas of the very earliest 

 Palaeozoic periods were poorly charged with calcic constituents, 

 and that they were thinly tenanted by lime-elaborating organ- 

 isms. Was the latter consequent on the former? 



The Cambrian rocks, whether occurring in North America, 

 on the continent of Europe, or in the British Islands, consist of 



* Mollusks and other invertebrates in their larval condition have horny 

 shells. It seems extremely doubtful that there was much calcic matter of any 

 kind in the Cambrian trilobites. Stony corals have not yet been found in 

 the Cambrians. Archaocyathus Atlanticus (presumed to be a sponge), from 

 the Potsdam of the Straits of Belle Isle, Newfoundland, may be calcareous, 

 Stems of crinoids occur in the Potsdams, 



