PREFACE 



WHEN the President of the Johns Hopkins University 

 invited me to inaugurate the Lectureship founded in that 

 seminary by Mrs. George Huntington Williams in memory 

 of her husband, the distinguished and widely-regretted 

 Professor of Geology there, I gladly availed myself of 

 the opportunity thus afforded me of renewing personal 

 relationships with the geologists of the United States, and 

 of thus helping to draw closer the bonds of sympathy 

 which unite the students of nature in the Old World and 

 in the New. 



In making choice of a subject that would be appro- 

 priate for the required course of lectures, I was influenced 

 by the announcement that geologists from all parts of the 

 States and from Canada would be asked to meet me in 

 Baltimore. As my audience might thus include repre- 

 sentatives of every department in the wide domain of 

 geological science, it was obviously desirable not to select 

 for treatment any limited field in that domain, but rather 

 to choose some topic of general interest in which students 

 of every part of the science might meet as on common 



