354 J. F. KEMP STRUCTURAL AND PETROGRAPHIC GEOLOGY 



experience reveals. There are cases of great interest involving joints, 

 dikes, sheets, the chilled border of intrusives, the contact zones, the 

 lenticular form of sediments and of ore bodies, the behavior of under- 

 ground waters, and many other subjects. 



The course was an endeavor to connect up geology with life, and, I 

 may truthfully add, it aroused interest among the members of the class 

 in an unexpected degree. Discussions among themselves and with the 

 instructor followed without end, and the young engineers or future geol- 

 ogists went out into the practice of their profession not entirely as if 

 into an unexplored and uncharted wilderness. We may not unreasonably 

 hope that even those who entered the engineering professions retained a 

 goodly proportion of the true scientific spirit, since it is no discredit to 

 any branch of scientific work to be useful. Accuracy and care in obser- 

 vation and soundness and skill in interpretation are demanded by both. 



PETROGRAPHIC INTERPRETATION AS TAUGHT BY PROFESSOR BEREEY 



The use of the word interpretation brings up another line of instruc- 

 tion and discussion, which has been developed in an impressively suc- 

 cessful way by Prof. Charles P. Berkey, for whom, on account of enforced 

 absence in the southwest, I am substituting in this symposium. Professor 

 Berkey has developed a course which deals with problems in petrography. 

 Now, the matter of interpretation is one of the most important parts of 

 the training of investigators after they have passed their novitiate in a 

 subject. Dr. Berkey has applied this principle in carrying his students 

 in petrography a stage beyond the systematic exposition of rocks as cov- 

 ered in the usual course. Students in this branch must necessarily be 

 trained on good, fresh typical specimens and sections, and must thereby 

 establish in their minds types and standards. Later on, however, when 

 they go into the field and are confronted with, weathered, altered, or 

 recrystallized exposures, they have to deal with rocks which are entirely 

 different; yet they must determine and interpret such rocks, whether 

 they write a scientific paper or apply what they have learned in the 

 service of an employer. Wall rocks of ore bodies, for example, are sel- 

 dom in a fresh condition and are usually so altered by thermal or other 

 processes as to be very different things from the specimens and sections 

 studied in the petrographic laboratory. A young observer, even though 

 carefully trained,' may find himself face to face with obscure and difficult 

 problems, of whose very existence he had not previously known. 



Moreover, ores are extremely interesting and fruitful subjects for 

 microscopic study. It may be of vital importance, in connection with 

 development in depth, to know whether an ore is original and primary 



