360 The American Geologist. June, 1892 
hardly supposable in the light of the constancy of chemical 
affinities. 
We are driven back therefore to the same long-contested field 
for which the neptunists and the piutonists have contended since 
the days of Werner. 
_ In recent years a new phase has been put on this controversy. 
Like some other long-disputed questions it appears that it is likely 
that there is truth on both sides, and that complete harmony 
may be effected, perhaps, by a little compromising acknowledge- 
ment of wrong by both of the adversaties—or at least by the 
acknowledgment of right in each. There has grown up a more 
perfect knowledge of the nature of rocks since the microscope 
has been introduced into lithology. Various differentiating 
characteristics have been established by which an unquestioned 
classification can be predicated, thus determining the origins of 
some rocks which before had been problematic. At the same 
time more careful field-work has been conducted in the crystalline 
rocks. Their major field-structures have first been correctly de- 
lineated. On this as a basis the microscope has laid out its 
minuter subdivisions, in a manner somewhat like the minuter 
classifications established by the paleontologist. The field-struct- 
ures must be determined first, for it is not possible for either 
microscopy in the crystalline rocks or paleontology in the later 
rocks to take independent initial cognizance of this. Their data 
are of subordinate importance,and are of value only as they can he 
grounded on foreknowledge of the actual structural features. 
Whether the Olenellus fauna preceded or followed the Paradoxides 
fauna in America could not be decided till a field examination was 
made in Newfoundland. The strata there revealed the Olenellus 
fauna anterior to the Paradoxides fauna, thus in effect reversing the 
conclusions which had been reached through paleontological rea- 
soning alone. In the same way the fact that a rock is eruptive is 
obtained in the first instance by field evidence—never by micro- 
scopic examination. Once the microscopic characters of a known 
eruptive rock being ascertained, those characters are good for the 
indication of a problematical eruptive rock so far as those known 
characters extend, but no further. If the known characters fail, 
and especially if adverse characters are brought to light, it is the 
part of caution, as well as of wisdom, to withhold a decision as 
to the origin of such a rock until new light can be reached. 
