146 CA CORTON 
most common types bordering continental coasts are “‘blue mud” 
and ‘‘green mud.”? Examination of the samples shows that the 
muds have been subjected to chemical changes on the sea bottom, 
but that they retain their original characters sufficiently to show 
that the material has been derived from the land. 
As a rule they are heterogeneous from the admixture of larger or smaller 
rock and shell fragments. ... . Rock fragments and mineral particles may 
make up as much as 75 per cent in some cases, the most characteristic species 
being quartz; the usual proportion of mineral particles is about one-fourth of 
the whole deposit. Amorphous clayey and muddy matters are always abun- 
dant, the average percentage being about 60, generally increasing in amount 
with greater distance from the land. 
Deltas and a more or less continuous shelf of typical form may 
be produced artificially on a small scale in a laboratory experiment, 
the apparatus for which has been described,” but, while such experi- 
ments serve admirably their purpose of illustration, it is obvious 
that quantitative results bearing on the relation of the depth of the 
edge of the shelf to wave-base would be difficult to obtain. As 
Davis remarks, “‘In most problems of geology and geography 
experiments have rather an illustrative than a demonstrative 
value.”’ 
It has often been pointed out that the edge of the continental 
shelf is situated everywhere (off exposed coasts) at a constant 
depth of about too fathoms, that is to say, at the greatest depth to 
which the water is stirred by wave action. Asa matter of fact the 
100-fathom line is situated generally at about the center of a convex 
curve to which the gentle slope of the continental shelf is tangent 
and which passes into the more steeply inclined surface of the con- 
tinental slope. The slope begins to steepen usually from a depth 
of about 70 fathoms, at which depth it would seem that the effects 
of wave action are becoming extremely feeble. 
This constant depth of the “‘continental edge,” together with the 
fact that a shelf borders, with rare exceptions, all the coasts of the 
world whatever their origin, whether by regional uplift or subsi- 
dence, by warping or by faulting, is of great significance (see Fig. 3). 
* Murray, The Ocean, p. 203. 
2R.S. Tarr and O. D. von Engeln, ‘“‘Representation of Land Forms in the Physi- 
ography Laboratory,” Journal of Geography, VIL (1908), 73-85. 
