PBEFATOEY NOTE IX 



SO greatly within short distances, and hence that a residual clay 

 cannot be relied upon to represent the residue from rocks of 

 the same nature immediately underlying, will serve to explain 

 in part the author's limiting himself so largely to a discussion 

 of massive eruptive materials. It is pleasing to note that later 

 analyses, by other and perhaps better workers, have fully cor- 

 roborated the results first obtained. 



In the preparation of the revised edition many errors have 

 been corrected, matter that proved non-essential eliminated, 

 and a considerable number of new analyses and illustrations 

 introduced. 



As will be readily perceived by those at all acquainted with 



the general literature, the publications of the U. S. Geological 



Survey, the U. S. National Museum, and the Bulletins of the 



Geological Society of America have been drawn upon to furnish 



materials for illustration. The writer, as before, is under special 



obligation to Dr. Milton Whitney of the U. S. Department of 



Agriculture for many of the mechanical analyses given, and to 



Professor L. H. Merrill of the Maine Experiment Station for 



numerous criticisms and suggestions. 



GEOEGE P. MERRILL. 



U. S. National Museum, January, 1906. 



