FISHING-GROUNDS 69 



it sinks from 22| to 27 J fathoms, is characterised by flat 

 schistous rocks. The third, some 7 or 8 miles wide, and 

 from 22^ to 30 fathoms below the surface, is an immense 

 plain of shell sand, slightly mixed with ooze towards the 

 south. The fourth zone, which extends to the extreme 

 limit of the plateau, or the loo-fathom line, is an immense 

 rocky plain. 



Let us double Cape Blanco and enter Greyhound Bay. 

 On the sill there is a shell-sand, more or less rich in 

 greenish ooze, which is found in the midst of a yellowish 

 shell sand, which is often covered with a thick bed of 

 dark green ooze. A great part of this ooze is swept by 

 the tidal current, which deposits it in a fan-shaped area 

 at the mouth of the bay. To the south of the bay the 

 Atlanto-Saharan bank continues to within a dozen miles 

 of St. Louis. The floor of the sea consists of sandstones, 

 scored by little valleys (mariscots) with gently sloping 

 sides and filled with shell sand as well as with small 

 quantities of ooze carried by the Senegal. The mariscots 

 constitute excellent fishing-grounds. 



