/. D. Dana on American Geological History. 307 



Divisions of time subordinate to the great ages will necessarily 

 depend on revolutions in the earth's surface, marked by abrupt 

 transitions, either in the organic remains of the region, or in the 

 succession of rocks. Such divisions are not universal. Each 

 continent has its own periods and epochs, and the geologists of 

 New York and the other States have wisely recognized this fact, 

 disregarding European stages or subdivisions. This is as true a 

 principle for the Cretaceous and Tertiary, as for the Silurian and 

 Devonian. The usurpation of Cromwell made an epoch in 

 English annals ; not in the French or Chinese. We should study 

 most carefully the records, before admitting that any physical 

 event in America was contemporaneous with a similar one in 

 Europe. The unity in geological history is in the progress of 

 life and in the great physical causes of change, not in the succes- 

 sion of rocks. 



The geological ages, as laid down by Agassiz, are the follow- 

 ing: — I. The Age of Fishes, including the Silurian and Devo- 

 nian; II. The Age op Eeptiles, embracing from the Carbon- 

 iferous through the Cretaceous; III. The Age of Mammals, 

 the Tertiary and Post-tertiary ; IY. The Age of Man, or the 

 recent era; — -fishes being regarded as the highest and characterisic 

 race of the first age ; reptiles of the second ; and mammals of the 

 third. 



More recent researches abroad, and also the investigations of 

 Prof. Hall in this country, have shown that the supposed fish re- 

 mains of the Silurian are probably fragments of Crustacea, if we 

 except those of certain beds near the top of the Silurian ; and 

 hence the Age of Fishes properly begins with the Devonian. 

 What then is the Silurian ? It is pre-eminently the Age of 

 Mollusks. 



Unlike the other two Invertebrate sub-kingdoms, the Radiate 

 and Articulate, which also appear in the earliest fossiliferous 

 beds, the Molluscan sub-kingdom is brought out in all its grander 

 divisions. There is not simply the type, but the type analyzed 

 or unfolded into its several departments, from the Brachiopods 

 and Bryozoa up to the highest group of all, the Cephalopods. 

 And among these Cephalopods, although they may have been 

 inferior in grade to some of later periods, there were species of 

 gigantic size, the shell reaching a length of ten or twelve feet. 

 The Silurian is therefore most appropriately styled the Mollus- 

 can Age. 



The Palaeozoic Trilobites belong to the lower tribe of Crusta- 

 cea, and Crustacea rank low among Articulates, Moreover, Crus- 

 tacea (and the Articulata in general) did not reach their fullest 

 development until the Human Era. 



The Kadiata were well represented in the Silurian periods ; 

 but, while inferior to the Mollusca as a sub-kingdom, only corals 



