2yo T//e Aineriviiii Utoloyist. NoM-inbcr, is% 
classilk'at ion. \\\ i\w application ortiie |)riiiciplc lo the region 
that was under consideration it was shown that unconformi- 
ties were the most important of all criteria in resolving into 
its grander subdivisions a vast sequence of ci'ystalline rocks 
which, as in the case of other similar masses, had defied all 
attempts at satisfactory arrangement and correlation. Had 
lie not been so untimel}^ called from his field of labor he 
might have expanded his theme so as to be of much wider, 
if not of universal application. It is not that Irving was the 
lirst to suggest the use of unconformities in delimiting the 
grander geological formations, for this at the present time is 
essentially the real foundation of our accepted geological clas- 
sification. Othercriteria, however, have so overshadowed this 
one that the fact of its ever having assumed an important role 
is well-nigh lost sight of entirely, and consequently the phys- 
ical breaks in stratigraphical succession excite little atten- 
tion, except as interpi'eted b}' fossils. Their true significance 
is now very nearly, if not completely, overlooked. In the ab- 
sence of fossils Irving was actually driven to the use of purely 
physical methods in dealing with the metamorphosed rocks, 
for any attempt to arrange the latter systematically, excej)t 
iilion faunal grounds, had been given up as useless. In other 
regions many writers before him had considered the phenom- 
enon of marked discordant sedimentation as a structural fea- 
ture and had actually gone so far as to regard unconformities 
as not only of regional but even of intercontinental extent. 
On the other hand, there were a ver}^ large number who be- 
lieved that unconformities at best Avere only local phenomena 
and therefore of small importance in stratigraphy. It was 
Irving's particular mission to determine how far unconformi- 
ties could be relied ui)on in a limited district, to point out 
clearly that in some cases they were of very wide, and in other 
cases of very limited extent, and in the geological classification 
of the non-fossiliferous rocks of a whole province, to propose 
a plan in which unconformities occupied a prominent place. 
Since Irving's time, short though the period has been, there 
has sprung into existence a new department of geological iu- 
([uiry, that not only reads later geological history in the geo- 
graphic forms presented, l)ut gives an entirely new insight 
into the real significance of unconformable relations between 
