Roland Duer Irriiig — Chamherlln. 5 
the U. S. geoloo;ical survey, ''On the Greenstones of the 
Menominee and Marquette Regions," by Dr. G. H. Williams. 
During these later years in which he was chiefly engaged 
upon monographic studies, he published numerous special 
papers, among which the more important were, "On the Nature 
of the Induration of the St. Peter's and Potsdam Sandstones, 
and of certain Archa3an Quartzites in Wisconsin,"* ''Para- 
morphic Origin of the Hornblende of the North-western 
States,"t "On Secondary Enlargements of Mineral Fragments 
in Certain Rocks," ;|: (jointly with professor C. R. Van Hise), 
and "The Junction between the Eastern Sandstone and the 
Keweenawan Series on Keweenaw Point,"§ (^jointly with presi- 
dent Chamberlin). 
Professor Irving's greatest contributions to science lay in the 
department of structural geolog}^ and genetic petrography. 
His investigations upon the great copper and iron-bearing 
series and the adjacent formations of the lake Superior region, 
constitute a contribution of the first order. The deep sym- 
pathy of the present writer with professor Irving's views on 
questions that have been subjects of divergence of opinion 
should perhaps restrain him from a full expression of his ap- 
preciation of the profound value of this work lest a color of 
personal partiality be thrown over this sketch, but it is not too 
much to assert that supporter and opponent alike recognize the 
ability which has characterized these investigations and the 
high order of value which must attach to them whatever inter- 
pretations may finally prevail. 
In the line of petrographic genesis professor Irving made 
two very notable contributions, first, the demoustratiou of the 
prevalence and importance of the secondary growth of certain 
fragmental constituents of clastic rocks and the crystallo- 
graphic co-ordination of the additions with the nuclear parti- 
cles. The existence of such a second growth in quartz grains 
was an earlier discovery of others but was hit upon by him inde- 
pendently. Jointly with his co-laborer, professor Van Hise, he 
demonstrated a similar second growth of hornblende and other 
*Am. .Jour. Sci., vol. xxv, p. 401, 1883. 
tAm. Jour. Sci., vol. xxvr, p. 321, 1883. 
JU. S. Geol. Survey, Bulletin No. 8. 
§ir. S. Geol. Survey, Bulletin No. 23. 
