EXPERIMENTAL GEOLOGY 671 



Page 

 Pre-Cambrian structure of the northern Black Hills, South Dakota, and its 



bearing on the Homestake ore body [discussion] ; by Sidney Paige 704 



Petroleum fields of northeastern Mexico between the Tamesi and Tuxpan 



rivers [discussion] ; by I. C. White 706 



Inadequacy of the sapropelic hypothesis of the origin of coal [abstract] ; 



by Edward C. Jeffrey 706 



Water and volcanic activity [discussion] ; by Arthur L. Day and E. S. 



Shepard 707 



Geologic section of the Panama Canal zone [abstract] ; by Donald F. Mac- 

 Donald 707 



Geological light from the Catskill Aqueduct [discussion] ; by Charles P. 



Berkey 711 



A classification of marine deposits [discussion] ; by A. W. Grabau 711 



Postglacial earth movements about Lake Ontario and the Saint Lawrence 



Valley [discussion] ; by J. W. Spencer 714 



Nature of the substance known as mother of coal and its relation to the 



process of coal formation [abstract] ; by Edward C. Jeffrey 715 



Potts ville-Allegheny boundary in the Interior Province [abstract] ; by 



David White 716 



Origin of the hard rock phosphate deposits of Florida [abstract] ; by E. H. 



Sellards 716 



Alteration processes and products within the Grenville limestone [ab- 

 stract] ; by Alexis A. Julien 717 



New light on the Keweenawan fault [abstract] ; by A. C. Lane 718 



Glacial erosion in the Genesee Valley system and its bearing on the Tertiary 



drainage problem of eastern North America [abstract and discussion] ; 



by A. W. Grabau 718 



Roots in the underclays of coal [abstract] ; by David White 719 



Objects and methods of petrographic description [abstract] ; by Charles P. 



Berkey 719 



EXPERIMENTAL GEOLOGY, ONE OF THE LARGE SUBDIVISIONS OF GEOLOGY 



BY FRED E. WRIGHT 



(Abstract) 



Experimentation has long been employed in geology as an indirect means of 

 approaching the solution of certain problems by a process of analogy; such 

 initiative experiments have met, however, with only a moderate degree of 

 success, and in recent years have been largely replaced by direct measurements 

 on reproducible systems of definite composition subjected to known conditions 

 of temperature and pressure. This change in mode of attack has been made 

 possible by the rapid development, during the past three or four decades, of 

 physics and chemistry ; this in turn has led to a better appreciation of the 

 fundamental principles involved in many geological problems and has placed 

 at the disposal of the geologist methods of attack which enable him to solve 

 problems hitherto deemed beyond the range of experiment. This part of 

 geology is not usually presented in the text-books, but its importance is so 

 great that it merits detailed treatment and may properly be considered a dis- 



