. Carnegie Institution of Washington publication. 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 relat
. Carnegie Institution of Washington publication. 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 ins
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