Feb. 1832. burnished rocks. 13 



They occur only within the limits of tidal action ; and as the 

 rivulet slowly trickles down, the surf must supply the polish- 

 ing power of the cataracts in the great rivers. In the same 

 manner, the rise and fall of the tide probably answers to 

 the periodical inundations ; and thus the same causes are 

 present under apparently very different circumstances. The 

 real origin, however, of these coatings of metallic oxides, 

 which seem as if cemented to the rocks, is not understood ; 

 and no reason, I believe, can be assigned for their thickness 

 remaining constant. 



One day I was amused by watching the habits of a Diodon, 

 which was caught swimming near the shore. This fish is well 

 known to possess the singular power of distending itself into 

 a nearly spherical form. After having been taken out of water 

 for a short time, and then again immersed in it, a considerable 

 quantity both of water and air was absorbed by the mouth, 

 and perhaps likewise by the branchial apertures. This pro- 

 cess is effected by two methods ; the air is swallowed, and 

 is then forced into the cavity of the body, its return being 

 prevented by a muscular contraction which is externally 

 visible; but the water, I observed, entered in a stream 

 through the mouth, which was wide open and motionless : 

 this latter action must, therefore, depend on suction. The 

 skin about the abdomen is much looser than that of the 

 back; hence, during the inflation, the lower, surface becomes 

 far more distended than the upper ; and the fish, in conse- 

 quence, floats with its back downwards. Cuvier doubts 

 whether the Diodon, in this position, is able to swim; 

 but not only can it thus move forward in a straight line, 

 but likewise it can turn round to either side. This latter 

 movement is effected solely by the aid of the pectoral fins ; 

 the tail being collapsed, and not used. From the body 

 being buoyed up with so much air, the branchial openings 

 were out of water ; but a stream drawn in by the mouth, 

 constantly flowed through them. 



The fish, having remained in this distended state for a 



