92 



PROCEEDINGS OF THE ACADEMY OP 



its occurrence. To Dr. F. A. Genth I am also indebted for allowing me to make 

 the investigation in his laboratory. The Guano rock is found on Islands N. E. 

 and N. of the coast of Venezuela in the Caribbean Sea, and belonging to that 

 Republic ; the islands form groups or keys composed of one hundred or more 

 small islands, inhabited principally and indeed almost solely, by water fowl, 

 who resort there in immense numbers for laying their eggs. Mr. Cassin informs 

 me that the birds are those commonly known as Gulls, Pelicans, and Cormo- 

 rants ; these have been the instruments by which the immense deposits of 

 Guano have been formed. But what the process has been is a problem not yet 

 fully solved. 



The Columbian Guano is a hard rock, consisting of two distinct portions : 

 the outer exterior crust, consisting of a white, frequently reniform coat- 

 ing, which in places where it is not decomposed has a polished surface 

 not unlike enamel ; it is composed of concentric layers, each of which 

 is about two or more lines in thickness. The entire outer crust forms but 

 a very small proportion of the whole rock ; a carefully selected portion of this 

 has been analyzed, which had not undergone any apparent decomposition, and 

 upon which the enamelled surface was well preserved. In some portions the 

 reniform surface is distinctly marked, and it resembles to a certain extent in its 

 appearance the menilite from Menil Montant near Paris ; the concentric layers 

 composing it are well marked, and when they are fractured or cut across, an 

 appearance is presented not unlike some varieties of agate. 



The following is the result of analysis : — 



(I)- 



From the oxygen ratio of this it appears that the Phosphoric Acid and Lime 

 exist in the proportion of SCaOPOs. The crucible after ignition showed indi- 

 cations that Chloride of Iron had been volatilized. 



Below this exterior crust the rock has a varied appearance ; it is of a dark 

 brown color, in places quite black, varying in others to a lighter shade, in 

 which portion are frequent cavities filled with minute crystals of gypsum, 

 readily recognizable with a pocket lens. The darkest portion is the most solid, 

 with a slightly vitreous lustre, and of a much greater tensibility and hardness 

 than any other portion of the guano ; it has a sub-concoidal fracture, sometimes 

 splitting into thin fragments, which are translucent, through some portions of it 

 1 have observed frequently, thin irregular bands, l-32d to l-16th of an inch 

 in width, which has the appearance of carbon. 



This portion was selected for analysis with particular care, with the view to 

 ascertain its exact composition; it was with difficulty reduced to the finest pow- 

 der and treated with distilled water (in the cold) in a beaker glass, being fre- 



[March, 



