MEMOIR OF JOHN HENRY CASWELL 
355 
without the advantages of foreign training. Practically the only published 
work before this time is contained in the observations of A. A. Julien on the 
rocks of Wisconsin and in the papers of E. S. Dana on the traps of the 
Connecticut Valley. Microscopes were far less convenient instruments 
than they have since become, and the study of the optics of crystals and the 
use of polarized light, always a difficult subject, presented exceptional 
obscurites. Nevertheless Mr. Caswell set to work, and, with the sympa¬ 
thetic aid of Professor Egleston and Dr. Julien and with the help of Dr. 
Waller in the preparation of chemical analyses, became, as the results prove, 
extremely skillful and accurate. 
It is also worthy of remark that at this time the headquarters of the 
Fortieth Parallel Survey were in New York, so that Mr. Caswell found 
himself in association with his old friend and fellow-student at Freiberg, 
Mr. Arnold Hague. In the preface of the sixth volume of the reports of 
this survey, Professor Zirkel, in addressing Clarence King, speaks as follows: 
“I cannot fail to gratefully acknowledge how much invaluable assistance 
I owe to you and to your excellent fellow-workmen, Messrs. S. F. Emmons 
and Arnold Hague. You well remember that happy time in New Aork 
when for many weeks we made together the preliminary examination of that 
vast collection of rocks, you had gathered under such difficulties, but with 
such eminent geological taste.” We can well imagine the enthusiasm with 
which this subject was taken up, under the stimulus of Professor Zirkel’s 
personality, since there are few teachers who have become so universally 
esteemed and beloved as himself. Under these circumstances the study of 
the Black Hills rocks was begun by Mr. Caswell, and his report furnishes 
one of the most important chapters in this invaluable work. The collection 
embraced a series of rhyolites, trachytes and, what was of extraordinary 
interest at the time, phonolites, the first of this rare type to be identified in 
America. Noav, nephelite, the diagnostic constituent of phonolites, is one 
of the most elusive of the more important rock-making minerals and time 
and again in these early years had either slipped by the older observers or 
else had been confounded with apatite. It was, however, correctly deter¬ 
mined by Air. Caswell and its recognition enabled him to describe and illus¬ 
trate this new and interesting occurrence. 
There is one other feature of Air. Caswell’s report which demands men¬ 
tion, and that is found in the plates, which were based upon his drawings 
from the microscope. They are of singular fidelity and beauty. Although 
prepared at so early a date they have been rarely if at all surpassed in later 
years. 
The work upon all branches of the Black Hills geology was delayed in 
publication by the unfortunate jealousies then prevailing among the four 
