so THE HANCOCK PACIFIC EXPEDITIONS VOL. 3 



which to attempt to evaluate or to describe the influences of air 

 drainage in the region, although there must be considerable impor- 

 tance in transportation of dust or soil particles alone. Therefore, the 

 hydrographic and biologic conditions of the northern section should 

 be essentially resultants of the combined influence of the river, the 

 ocean, and atmospheric phenomena. Although the gradient in the 

 upper sixty miles of the section is well established, the trough is 

 partly filled by Angel de la Guardia, Tiburon, and other islands, and 

 it lacks the appearance of a broad, open, south tilted basin 

 characteristic of the remainder of the Gulf. 



Twenty-five stations were occupied in the northern section, more 

 than one catch being made at several of them. The total of catches 

 was thirty-eight. (See Figure 1 for route and location of stations in 

 the section.) Of these catches only seventeen were too small to be 

 significant, and seven of them were at six stations fairly near shore 

 in the southwest part of the section. The other ten were at stations 

 irregularly distributed up to and in the neighborhood of Angel de la 

 Guardia Island. Five stations yielded six large catches, two between 

 Angel de la Guardia Island and the west shore of the Gulf, three 

 just south of Tiburon Island, and one in mid-Gulf about seventy 

 miles to the southeast of the latter. The first five were taken in rela- 

 tively shallow water, the last at a point where the depth was more 

 than a thousand meters. Such a distribution of large catches suggests 

 that the northern section as a whole has a tendency to good produc- 

 tiveness. Nine of the catches of moderate significance were scattered 

 along the route in the section. Six others were obtained at two sta- 

 tions at the north end of Angel de la Guardia Island. 



While the locations of these catches of moderate importance were 

 such as to support the suggestion from larger catches that the section 

 as a whole is productive, at least on certain occasions, consideration 

 of the three large catches at Tiburon Island leads to the idea that the 

 region of these islands is more than ordinarily favorable for produc- 

 tion of plankton diatoms as far as the Gulf is concerned. Perhaps 

 the islands mark the most favorable point of mixing of river, Gulf, 

 and ocean influence. In 1921 (Allen, 1923) this section was visited 

 in April and again in June by the expedition of the California 

 Academy of Sciences. On both occasions large catches were found 

 not only near Angel de la Guardia Island but also at points much 

 farther up in the Gulf, one (at Georges Island) being only about ten 



