SEPTEMBER 29, 1899. ] 
prominent geographic features that are situated 
within their borders. 
2. Topographic Expression.—While the sur- 
face relief, or topographic expression, of various 
regions has received more or less considera- 
tion in the past, its great variety and character- 
istic types have been only vaguely connected 
with the structure and composition of the un- 
derlying rocks. Only recently has the inter- 
pretation of topography found a philosophical 
foundation. Only within the last few years 
has arisen a entirely new department of geolog- 
ical inquiry. The rapid development of this 
new science of geomorphy, or geographic geol- 
ogy, has given a new meaning to stratigraphy, 
and therewith has furnished a reliable criterion 
for determining and mapping geological forma- 
tions that was wholly unthought of before. 
General topographic expression may be now re- 
garded as one of the most important and dis- 
tinctive attributes of geological terranes. 
When its topographical type and peculiarities 
are clearly discerned a terrane may be, with 
frequent checks from other sources, traced many 
miles with rapidity and certainty, from horse- 
back, or often even from the swiftly moving 
railway train. 
As the differences in the topography of a 
region are dependent primarily on the rela- 
tive power of resistance to erosion that the sey- 
eral contiguous layers possess, there is usually 
a close relationship existing between this fea- 
ture and the other characteristics which have 
been heretofore generally considered alone in 
connection with stratigraphy. Thus, an exten- 
sive shale bed will often be worn down to a 
lowland plain, while the limestone or sandstone 
strata on either side of the belt will form ridges, 
The faunas in shales are usually peculiar to 
them, and very distinct from lithologically dif- 
ferent beds. Again, a limestone which forms 
the hard member with the soft shales is, when 
intercalated with crystallines, itself the soft 
member and constitutes the valleys. 
Although the same kinds of beds may produce 
directly the opposite phases of topographic ex- 
pression, for each particular region the phase 
assumed is distinctive and generally extends 
throughout the geographical extent of the ter- 
rane. 
SCIENCE. 
457 
3. Lithologic Nature.—In the early part of the 
century it was customary to regard lithological 
characteristics as the most important features in 
the recognition of geological formations. In 
correlating deposits more or less widely sepa- 
rated, this character was depended upon, to the 
exclusion of all others. Ata later date, when 
other criteria were applied, the determinations 
which had been made upon purely lithological 
grounds were found to be so faulty and unreliable 
that the use of’ this feature finally came to be ig- 
nored almost altogether. Of late, the real value 
of the lithologic factor is beginning to be more 
fully appreciated. It is certainly as_ trust- 
worthy as the faunal characteristics of a ter- 
rane, and, in addition, is generally of wider 
application. 
The lithology of a geological formation, out- 
side of the massive rocks, is largely a function of 
the attitude of the adjoining land areas at the 
time that the beds were laid down. Hence, 
there is a close connection between the local 
character of the forming strata and the position 
of the adjacent land as changing under the in- 
fluence of diastatic movement. In noting the 
distinguishing characters of a terrane the lithol- 
ogy should receive the fullest consideration and 
the most careful discrimination. 
4. Stratigraphic Delimitation.—Until very re- 
cently little attention has been paid to the exact 
vertical or range limits of geological terranes. 
In an indefinite and incidental way they have 
been often fixed within narrow bounds, and the 
local features explained for the typical locality ; 
but farther than this most descriptions are 
stratigraphically inexact. 
The determination of definite, easily recogniz- 
able upper and lower horizons, that are readily 
traceable over considerable areas, are of prime 
importance, not only to havea compact, natural 
unit, but on account of presenting reliable 
features for the correlation in different parts of 
the geologic province. The division line be- 
tween terranes is not always equally distinct 
and prominent throughout the areal extent of 
the deposit, and different criteria have often to 
be resorted to in different parts of a province. 
While the exact position of a terrane in the 
general geological scale is not always to be 
made out with exactness at first, its approxi- 
