104 CORAL-REEFS. 



belonged ; but from the numbers experimented on, it is 

 certain that all the more conspicuous kinds must have been 

 included. Dr. Allan informs me, in a letter, that he 

 believes it was a Madrepora, which grew most vigorously. 

 One may be permitted to suspect that the level of the 

 sea might possibly have been somewhat different at the two 

 stated periods; nevertheless, it is quite evident that the 

 growth of the ten-pound masses, during the six or seven 

 months, at the end of which they were found immovably 

 fixed 1 and several feet in length, must have been very 

 great. The fact of the different kinds of coral, when placed 

 in one clump, having increased in extremely unequal ratios, 

 is very interesting, as it shows the manner in which a reef, 

 supporting many species of coral, would probably be affected 

 by a change in the external conditions favouring one kind 

 more than another. The growth of the masses of coral 

 in N. and S. lines parallel to the prevailing currents, 

 whether due to the drifting of sediment or to the simple 

 movement of the water, is, also, a very interesting circum- 

 stance. 



A fact, communicated to me by Lieut. Wellstead, I.N., 

 in some degree corroborates the result of Dr. Allan's experi- 

 ments : it is, that in the Persian Gulf a ship had her copper 

 bottom encrusted in the course of twenty months with a 

 layer of coral, two feet in thickness, which it required great 

 force to remove, when the vessel was docked : it was not 

 ascertained to what order this coral belonged. The case of 

 the schooner-channel choked up with coral in an interval of 



1 It is stated by Mr. de la Beche {Geological Manual, p. 143), on the 

 authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that 

 some specimens of Polypifers, placed by him in a sheltered pool of 

 water, were found in the course of a few days firmly fixed by the 

 secretion of a stony matter, to the bottom. 



