GENERAL LAWS IN GEOLOGY. 541 



provisionally termed protozoic, and the lower Silurian rocks may 

 probably be looked upon as its upper members. Below this, geolo- 

 gists place a hypozoic or azoic series of rocks. 



Geologists differ as to the question whether these changes in the 

 inhabitants of the globe were made by determinate steps or by insen- 

 sible gradations. M. Agassiz has been led to the conviction that the 

 organized population of the globe was renewed in the interval of each 

 principal member of its formations. 8 Mr. Lyell, on the other hand, 

 conceives that the change in the collection of organized beings was 

 gradual, and has proposed on this subject an hypothesis which I shall 

 hereafter consider.] 



Sect. 2. Transition to Geological Dynamics. 



WHILE we have been giving this account of the objects with which 

 Descriptive Geology is occupied, it must have been felt how difficult 

 it is, in contemplating such facts, to confine ourselves to description 

 and classification. Conjectures and reasonings respecting the causes 

 of the phenomena force themselves upon us at every step ; and even 

 influence our classification and nomenclature. Our Descriptive 

 Geology impels us to endeavor to construct a Physical Geology. This 

 close connexion of the two branches of the subject by no means in- 

 validates the necessity of distinguishing them : as in Botany, although 

 the formation of a Natural System necessarily brings us to physiolo- 

 gical relations, we still distinguish Systematic from Physiological 

 Botany. 



Supposing, however, our Descriptive Geology to be completed, as 

 far as can be done without considering closely the causes by which 

 the strata have been produced, we have now to enter upon the other 

 province of the science, which treats of those causes, and of which we 

 have already spoken, as Physical Geology. But before we can treat 

 this department of speculation in a manner suitable to the conditions 

 of science, and to the analogy of other parts of our knowledge, a 

 certain intermediate and preparatory science must be formed, of which 

 we shall now consider the origin and progress. 



Brit. Assoc. Report 1842, p. 83. 



