232 J. F. KEMP AFTER-EFFECTS OF IGNEOUS INTRUSION 



of some of the Fellowship and not beyond the personal participation of 

 a few who are still active. One recognizes at once the entrance into 

 general use of the polarizing microscope and the consequent widespread 

 study of rocks in thin sections. Although this method of investigation 

 was cultivated in this country in the decade of the seventies by A. A. 

 Julien, C. E. Wright, E. S. Dana, J. H. Caswell, George W. Hawes, 

 B. K. Emerson, M. E. Wadsworth, and perhaps one or two others; and 

 although stimulated by Zirkel's volume in the Reports of the Survey of 

 the Fortieth Parallel, by Hawes' invaluable descriptions of the rocks of 

 New Hampshire, and by Caswell's beautifully illustrated pages on the 

 rich soda rocks of the Black Hills, the early years of the next decade saw 

 the greatest impetus to American petrography. 



Whitman Cross, J. P. Iddings, J. S. Diller, F. D. Adams, George H. 

 Williams, J. E. Wolff, and one or two more, all returned from European 

 study fired with ideals and ambitions gained in the laboratories of Zirkel 

 or Rosenbusch. George H. Williams became the preeminent teacher, and 

 his contagious enthusiasm, clear presentation, and temperament of the 

 artist in his work inspired great zeal in many younger men who sat in 

 his lecture-room or wrought in his laboratory. I think we may truthfully 

 say that the work of the men mentioned, of others who came later, and 

 of contemporaries abroad has rounded out the descriptive part of the 

 branch of geology which deals with the igneous and metamorphic rocks. 

 We are not likely to discover anything further which is very essential 

 regarding components and structures. The actual facts are well and 

 completely recorded and lengthy papers elaborately descriptive of details 

 are not able to add much to the general stock of knowledge. They have 

 become wearisome and of slight value. With the sediments the case is 

 different. Long neglected, they are now gaining recognition, and for 

 some years we are likely to learn much that is new and significant con- 

 cerning the minute mineralogy of the clastic and organic rocks. With 

 all the rocks, description has largely given way to interpretation, petrog- 

 raphv lias become petrology, and the possibilities of fundamental advances 

 in our understanding of processes are now looming large. 



After-effects of igneous Intrusion 



Another subject, impressive in the importance which it has been gain- 

 ing in just about the three decades covered by the life of our Society, is 

 that which involves the after-effects of igneous intrusion, and it is to a 

 survey of these processes and results that I wish to direct your attention. 

 They arc fruitful ones and are large with the promise of clearing up 



