INTRODUCTION. 



The prime duty of a geological survey is to make a geological map 

 of the country. Those who are unfamiliar with the duties of a geolo- 

 gist are apt to suppose that no great amount of knowledge is needed 

 to produce a satisfactory map of this kind. Those who have tried it 

 know better. The field geologist is at once confronted by the theo- 

 retical aspects of his science in such a manner that he is compelled to 

 adopt at least tentative views. He must decide what is to be mapped, 

 and this decision implies that he knows or assumes relations between 

 the various members of the series with which he has to do. All geolo- 

 gists worthy of the name are continually and painfully aware that 

 they deal largely in uncertainties or matters of opinion, and thev 

 can not fairly be reproached with the insufficiency of the grounds 

 which they sometimes have to show for the views they adopt, unless 

 they lay themselves open to the accusation of neglecting results 

 established by theory and experiment. 



Geology is not a science, but the application of the sciences to the 

 elucidation of the history of the earth. Its best developed and 

 oldest branch is zoological geology or paleontology, and next in order 

 of development, though substantially the latest in chronological order, 

 is mineralogical geology as represented by petrographv. The rapid 

 advance in the description of rocks is due, as everyone knows, to the 

 introduction of the microscope and of exact optical methods in the 

 determinations of minerals. Less advance has been made in the wider 

 subject called petrology or lithology, as well as in orogeny, vulcanism, 

 and ore deposits. The resources of the terrestrial laboratory so far 

 transcend those which can be equipped by man that vast groups of 

 geological phenomena still await even approximate explanation. 



Observations on the lithosphere alone will not suffice to elucidate 

 these dark regions. As Messrs. Day and Allen very properly insist, 

 "geological field research is a study of natural end-phenomena, of 

 completed reactions, but with a very imperfect record of the earlier 

 intermediate steps in the earth-making processes." In fact, the un- 

 known quantities outnumber the equations which field observation 

 puts at our disposal. In their present state of development the 

 sciences of physics and chemistry can aid the geologist only to a mod- 

 erate extent. We do not know in most cases whether the laws of 



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