PREFACE. 



Every geologist is, probably, more or less misled by 

 theory. If the earth was washed and the rocks left clean, 

 they would not disagree in regard to rocks. But they are 

 now dependent on naked cliffs and deep river-washed ra- 

 vines ; which present to the eye less than a hundreth of the 

 evidence required. For ninety-nine hundredths, theory alone 

 furnishes the facts upon which the very same theory is found- 

 ed. But a long course of observatioiis and careful com- 

 parisons, have done considerable towards a correct system 

 •of generalization. 



Geology is subject to an evil peculiar to itself. If its vo- 

 taries disagree, the common learner has neither time nor 

 inclination to review their data, by visiting localities refer- 

 Ted to, and thus to correct their mistakes and adjust their 

 differences. The reasons, fully written out, which govern 

 the experienced geologist, would require numerous octavos 

 for each stratum. Therefore the learner must rely upon his 

 confidence in his teacher's habits of careful investigation, 

 his fidelity, his independence, and his talent at generaliza- 

 tion. He must not overlook the advantages presented by 

 the district of country which he examines. should 

 therefore compare the districts examined by different geolo- 

 gists. 



The little island of Britain can furnish no authority for 

 a general system ; though the industry of distinguished ge- 

 ologists has done much towards an elucidation of important 

 points, in detail. France, taking in the Alps as a primi- 



