252 LEADING AMERICAN MEN OF SCIENCE 



scope, have their supreme significance as monumental inscriptions, 

 the deciphering of which may reveal to us the history of the earth. 

 Yet surely this conception had never been so clearly formulated, 

 and the whole treatment of the subject never so consistently ad- 

 justed thereto, as in the writings of Dana. The portion of pre- 

 vious manuals dealing with the distribution of the series of strata 

 had generally borne some such title as "Stratigraphical Geology" ; 

 and very commonly the series had been traced backward, com- 

 mencing with the most recent strata. 1 The phrase, "Historical 

 Geology," which forms the title of that part of Dana's Manual, 

 involves a distinct clarification of the general view of the science. 

 Starting with this conception, of course he deals with the earliest 

 formations first. In treating of each era, he endeavors to recon- 

 struct, from the evidence afforded by the kinds and distribution of 

 the rocks, the physical geography of the time. In accordance with 

 this general principle, the sections of the Historical Geology in the 

 Manual were not characterized as series, systems, and groups of 

 strata, but as eras, periods, and epochs of time. The common 

 use in recent geological writings of such phrases as "Silurian era," 

 rather than "Silurian system," etc., is a testimony to the influence 

 of Dana's mode of treatment. 2 The key-note of Dana's concep- 

 tion of geology as history is clearly sounded in his presidential 

 address before the American Association for the Advancement 

 of Science in 1855. The title of the address is significant On 

 American Geological History. In that address occurs the follow- 

 ing passage: "Geology is not simply the science of rocks, for 

 rocks are but incidents in the earth's history, and may or may 

 not have been the same in distant places. It has its more exalted 

 end, even the study of the progress of life from its earliest dawn 

 to the appearance of man; and instead of saying that fossils are 

 of use to determine rocks, we should rather say that the rocks are 

 of use for the display of the succession of fossils." 



To Dana we owe the formulation of a doctrine now almost 

 universally adopted by geologists the doctrine of the permanence 



1 As in the manuals by Lyell and De la Beche. 



2 Williams, in Journal of Geology, vol. 3, p. 606. 



