644 REVIEWS 



were changed into subsequent streams. As these developed, the 

 superimposed consequent streams became atrophied, leaving only the 

 lower part of the San Mateo Creek. "The remarkably straight valley 

 of Crystal Spring and San Andreas is a magnificent example of a 

 subsequent drainage system extending laterally along a fault in 

 opposite directions normal to the original consequent drainage." 

 Other excellent examples of captured and superimposed streams are 

 mentioned. 



Lake Merced, though in a structural valley, is considered a 

 drowned valley of stream erosion. The bottom of the lake is ten feet 

 below sea level, which demonstrates its recent submergence. Its access 

 to the ocean was cut off by sand dunes which dammed up its channel 

 till the water stood ten feet above tide. 



The contrast between the ocean shore-line and that of the bay is 

 noted. The former presents steep cliffs and sandy beaches due to the 

 vigorous action of the waves, while the latter has a tidal marsh, in 

 places some miles wide. This marsh may be due to the deposition of 

 material from the bay water during the rainy season. The fact that 

 the tidal marsh is perfectly level and the tidal streams reach back to 

 its rear, indicates that there has been no very recent uplift. On the 

 other hand there may have been a slow subsidence, during which the 

 rate of deposition was equal to the subsidence. 



The colored relief map accompanying the paper is worthy of 

 special note for its elegance and effectiveness. 



A. H. Purdue. 



