Feb. 1832. 



BURNISHED rocks: 



13 



They occur only within the hmits 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 



