148 CLAS CORTON 
It indicates that the present-day shelf has taken form since sea and 
land assumed their present relative levels,* and therefore that the 
molding of the shelf is geologically a very recent event; for most 
coastal lands afford evidence of considerable movements either of 
uplift or of subsidence having taken place at a not very distant 
date. 
Nansen,” reasoning from the same data, comes to a very different 
conclusion. While he recognizes the practical uniformity of level 
of the edge of the continental shelf, he argues from the assumption 
that the shelf is an ancient feature and makes the remarkable deduc- 
tion that, though great changes of level have taken place, which in 
at least some cases he regards as movements of the land rather than 
of the ocean, the continental margins have everywhere returned to 
their ancient level. 
A general idea of the width of the continental shelf is given by 
Fig. 3. More precise data may be obtained from an inspection of 
ocean charts upon which the 10o-fathom submarine contour line 
is or may be drawn, or more conveniently from the numerous maps 
in Stieler’s atlas which show the 200-meter line. Variation in 
width depends on several factors; no doubt the most important 
is the depth of water into which the shelf has grown outward since 
the latest movement of the strand. If the vertical movement has 
been small, as, for example, around the British Isles, the present 
shelf is a modified older shelf, added to at the margin, and therefore 
broad. 
Another important factor must be the presence or absence of 
abundant waste from the land, which will be largely determined 
by the presence or absence of large rivers in the vicinity. Still 
another must be the time that has elapsed since the latest important 
movement of the strand. 
Variations in the width of the shelf do not affect the question 
of its essential continuity, which, as pointed out above, prove its 
capacity for rapid growth and renewal. It is obvious that the 
«T. C. Chamberlin says of it: ‘“The terrace is as universal (at least in its initial 
stages) as the sea border and is a necessary consequence of the relations of sea and 
land”’ (Jour. Geol., VI [1898], 526). 
2 F. Nansen, ‘‘Oscillations of Shorelines,’ Geog. Jour., XXVI (1905), 604-9. 
