

Prof. B. Silliman, Jr., on Gibbsite and Allophane. 417 



Atoms. Calculated. Found. 



2 Si 1154-62 --= 24-211 22-654 



* n m 



3 Ai 0926-99 40408 38-768 



15 H 1687-20 35-421 35-240 



4768-81 100000 96 662 



2-833 magnesia 

 0*505 loss. 



100 000 



If the magnesia is regarded as an essential constituent of the 

 mineral, it gives a complicated and improbable formula, while it 

 is not at all remarkable that a mineral formed by infiltrating wa- 

 ters as this evidently is, should contain a notable amount of for- 

 eign impurities. 



I was struck with the resemblance which exists between these 

 two associated species and the stalactitic concretions of hydrated 

 aluminous silicates brought by Mr. Dana from the lava caves of 

 volcanic Islands in the Pacific. These I have long since examined, 

 and the results are contained in the Geological Report of the U. 

 S. Exploring Expedition, by Mr. Dana. 



There can no longer be any doubt of the identity of the Hy- 

 drargillite of Rose with Gibbsite ; — the former is the crystalline, 

 the latter the amorphous variety of the same species. It is not im- 

 possible that this same locality may furnish specimens of massive 

 Wavellite, and perhaps it was such an one which gave to M. 

 Hermann the result which has called out this article. The phos- 

 phoric acid of both Hydrargillite and Gibbsite is in too small 

 quantity to be regarded as an essential constituent 



In conclusion I would say, that on a former occasion, in ma- 

 king a cursory qualitative examination of Gibbsite, I detected* 

 silica in it, and so reported it to Mr. Dana, and the statement is 

 repeated in his smaller work. I have now no doubt that the por- 

 tion of the mineral then examined, was contaminated with a por- 

 tion of allophane — in fact, the examination was made upon the 

 same specimens, in which, by closer observation, I have since de- 

 tected the latter mineral. It requires some care to separate the 

 two entirely, and I have little doubt that the silica, mentioned by 

 Dr. Thomson in his analysis of Gibbsite, was derived from the 

 same intermixture of the two species. 



Analytical Laboratory, April 1, 1849. 



eries, VoL VII, No. 2L— May, 1849. 53 



