270 



PEOCEEDINGS OF THE GEOLOGICAL SOCIETY, 



the divergent, are enabled to recur by the hardiness of their consti- 

 tution. Many are continued in existence by their numbers and di- 

 minutive size ; others are washed dead into newer sediments, and are 

 therefore not really recurrent. Those animals, on the contrary, 

 which have weak vital powers, such as the Echinodermata, Zoojphyta, 

 and Bryozoa, scarcely excepting Graptolites, have little or no vertical 

 range ; and others, because they are fixed or sedentary. These re- 

 marks apply to all kno^vn Silurian regions, their recurrents being 

 often identical. One very important observation should be made, 

 and to the effect that, when recurrence in contiguous sedimentary 

 strata exceeds a certain limited amount, such strata cease to belong 

 to different epochs, and must be united, as has been done in Wales 

 by Sir Eoderick Murchison. 



Where there is much recurrency, we may infer that in such a 

 series of groups the conditions of the ocean were nearly stationary, and 

 that the mutual relations of sea and land remained much the same ; 

 for altered conditions are the great obstacle to recurrence. On this 

 subject, I shall only at present notice, as occasioning these altera- 

 tions, the secular oscillations and sudden uplifts which have always 

 from time to time created new coast-lines, thrown up banks of mud 

 and sand, and have changed the direction and force of currents. 

 These currents may bring with them colder or warmer, turbid or 

 heavily loaded waters ; they may sweep away beds of marine plants 

 with their inhabitants, and introduce new life ; or they may bury 

 old sea-bottoms beneath a thousand feet of shingle and conglo- 

 merate. 



Table XI. — Number of Silurian Species recurrent in one or more 



sets of strata. 



No. of G-roups in which 

 recurrence takes place. 



I 



II 



III 



IV 



V 



VI 



VII 



VIII 



Total. 



/ New York... 



Recurrent , 

 Fossils. 1 



(Wales 



93 



16 



6 



1 



1 



1 





3 



121 



129 



51 



14 



2 









... 



196 





By intermixing fossils which originate in different horizons, 

 recurrency produces a difficulty in determining the dates of such 

 horizons ; but this may usually be lessened by the presence of 

 other forms, and by the nature of the sediment. Any given 

 species occupies, as a general rule, the same horizon in all parts of 

 the globe. A fossil which is Upper Silurian in New York is the 

 same in Scandinavia or Wales, as we learn from the Tables of 

 De Yerneuil and D'Archiac (Geol. Trans. 2 ser. vol. vi.), and from 

 my Table of the fossils common to New York and Euroj^e. This is 

 a state of things due to the absence of isothermal lines until very 



