POPULAR SCIENCE MONTHLY 



deepened by earth movements, altlioiigh such is not apparent in 

 the many tributaries. It has been found that the greatest amount 

 of upward warping is in the mountain regions, and the greatest de- 

 pression is supposed to be along the margins of oceanic abysses. 

 Such movements would have a tendency to somewhat increase ter- 

 restrial and submarine declivities; but if transverse upward warping 

 became exaggerated, the valleys would become barricaded so as to 

 form basins, like that of Lake Ontario, or even greater sea basins. 

 Such, however, is not the case with the valleys crossing the sub- 

 merged coastal plains. 



Comparison of Land and Drowned Yalleys. — Since the sub- 

 marine valleys, wherever traceable to the shores, are found to be 



continuations of the 

 buried valleys crossing 

 the coastal plains, they 

 have considerable mag- 

 nitude where first de- 

 tected in the soundings. 

 Parenthetically it may 

 be stated that during 

 the late minor oscilla- 

 tions of the continent 

 the old valleys have be- 

 come filled with sand, 

 etc., for some distance 

 seaward ; but beyond 

 this fringe they are 

 always found where the 

 soundings have been 

 taken sufficiently near 

 together. Submerged 

 valleys gradually in- 

 crease in size until they 

 enter the oceanic em- 

 bayments indenting the 

 margins of the con- 

 tinental mass. These 

 embayments have been 

 found to vary from per- 

 haps ten to forty miles 

 in width, or within the 

 limit of size shown in the modern Mississippi and St. Lawrence Val- 

 leys. Even the upper caiion of the Colorado River has a width of 

 twelve miles. Consequently, the breadth of the drowned valleys 



Fig. 7. — Longitudinal section of the Cazonan channel 

 (south of Cuba), showing a similar but shorter fiord 

 dissecting the land mass, the submerged floor of which 

 is shown by the broken shading. 



Fig. 8. — Longitudinal sections of the Atozac Valley, de- 

 scending from the Mexican plateau (eight thousand 

 feet above the sea), on the same scale us that of the 

 drowned valleys, but which is here too small to show 

 the nxuncrous steps, except the mean declivity for 

 comparison with that of the abrupt slopes of the sub- 

 marine steps. 





Fig. 9. 



-Longitudinal section of a similar valley south of 

 Monterey in Mexico. 



