58 Professor Horsford on the 



The rock formed under water exclusively is composed of 

 grains of size less than that of a mustard seed, which to the 

 naked eye appear quite globular, and of uniform diameter. 

 More carefully examined with a microscope, they are found 

 to be far from regular in form or uniform in size, but pre- 

 sent numerous depressions and prominences. Distributed 

 throughout the intervening spaces is a fine deposit of carbon- 

 ate of lime, which adheres with considerable tenacity to the 

 surface upon which it rests. 



The surface or crust-rock, though not strictly homogeneous, 

 is composed of particles so minute as not to be distinguished 

 from each other. It dissolves in hydrochloric acid, leaving 

 a flocculent residue. The solution, when evaporated to dry- 

 ness, and ignited, readily redissolves in hydrochloric acid, 

 with only an occasional residue. The solution gives no pre- 

 cipitate with chloride of barium. Nitrate of silver gives, in 

 a nitric acid solution; a white precipitate, soluble in ammonia. 

 The aqueous extract gives to alcohol flame the characteristic 

 soda tint. The powdered rock, dried at 100° C, when heated 

 in a dry tube, gives off water. 



Thus the qualitative analysis of the incrusting rock shewed 

 it to consist of lime, soda, carbonic acid, hydrochloric acid, 

 water, and organic matter. There were also variable traces 

 of peroxide of iron, magnesia, and silica. The former two 

 were wanting in most of the specimens examined, and the 

 silica in some. Numerous specimens were examined for 

 alumina, without in any instance finding a trace of this sub- 

 stance.* 



* I examined, also, all the species of coral at my command, without finding 

 a trace of alumina in any of them. The hydrochloric acid solution of the coral 

 was precipitated with ammonia. The washed precipitate was digested for 

 several hours with potassa (previously tested for, and found to he free from, 

 alumina), and filtered. The filtrate was then neutralised with hydrochloric 

 acid, and ammonia added. After standing for several hours, there appeared 

 filaments which were soluhle neither in potassa nor nitric acid, and which, ex- 

 amined with the microscope, proved to he paper ; they had heen derived from 

 the filter. Beside these, there was no precipitate. The quantities employed 

 wore, in several instances, from a quarter to half a pound of material. There 

 were examined, Millepora alcicornis ; Meandrina lahyrinthica, two specimens ; 

 Manieina palmata ; Myccdia areolata ; A straw microcosmos, two specimens; 

 rock Bubaerial and rock submarine, numerous specimens. 



