383 
by Muddy Fork of Silver Creek. Such piracy will continue until a balanced 
condition of the gradients of the two stream systems is reached. Such a 
condition will mark the beginnings of old age of the stream systems. when 
stream adjustments are practically complete. It is further noted that in 
the adjustment of the barbed tributaries to the reversed drainage, an 
unusual set of bed-rock terraces is being made where the barbed tribu- 
taries join the main stream. These terraces are due to conditions unlike 
any which have elsewhere come under the observation of the writer. 
The special features in this paper to which the attention of physiogra- 
phers is directed are as follows: definition of the terms ‘‘geologie struc- 
ture” and “topographic condition” as physiographic terms; a grouping of 
the physiographic factors under two heads, active factors and conditioning 
factors; a division of the conditioning factors into the so-called “material” 
and “time elements”, with definitions and illustrations of the new terms 
used; a declaration of the importance of geologic structure (lithology and 
structure) in the development of regional topographic forms in the stages 
of youth and maturity ;* an extension of the use of the term “rejuvenation” 
in ihn the term “static rejuvenation” is proposed, and along with the 
term a regional example of it offered. These are phases of physiography 
which the writer attempts to make pertinent or which he wishes to present 
initially. Finally contributions to regional physiography are made in the 
treatment of a particular region as a whole and parts of it in detail. 
The viewpoint of this paper is pre-eminently that of explanation of phy- 
siographic phenomena. <A region is selected and discussed purposely for the 
presentation of this sort of physiographic treatment. The common physi- 
ographic forms and processes are given little space. It is held that the 
topographic map contributes such data as size, shape, and relationship of 
topographic forms, and that the text need not be filled with a mass of 
rather unnecessary and burdensome detail. The text should be econ- 
cerned primarily with the general conditions which permit of the develop- 
ment of the particular array of topographic forms, and should be foéused 
especially on the unusual forms and unusual relationships. When these 
latter things are considered the text may have in it then such additional 
descriptive matter as may be necessary in the explanation of the forms or 
relationships. Such a program is attempted in the presentation of the 
material in this paper. Attention is first centered upon the factors whieh 
have controlled the topographic development. Then follows a presentation 
of the unusual features with sufficient detail to show what the features 
are and why they exist. 
12This idea is by no means new, but it appears to the writer that too little 
emphasis has been placed upon it in physiographic papers. English physiographers 
are more appreciative in this respect than their American neighbors. ‘‘These forms 
(land forms) never occur scattered haphazard over a region, but always in an orderly 
subordination depending on their mode of origin. .... . . The geological structure 
and the mineral composition of the rocks are often the chief causes determining 
the character of the land forms of a region. Thus the scenery of a limestone country 
depends on the solubility and permeability of the rocks, leading to the typical Karst- 
formations of caverns, swallow-holes and underground stream courses, with the 
contingent phenomena of dry valleys and natural bridges. A sandy beach or desert 
owes its character to the mobility of its constituent sand grains, which are readily 
drifted and piled up in the form of dunes. A region where volcanic activity has 
lead to the embedding of dikes or bosses of hard rock amongst softer strata pro- 
duces a plain broken by abrupt and isolated eminences.”” Hugh Robert Mill, Enecyclo- 
paedia Britannica, Vol. XI, Eleventh Edition, P. 633. 
