332 I On Name and Hill — Solution of Metals. 



4. The velocity of a reaction at the actual boundary surface 

 between a solid and a liquid is not necessarily extremely rapid, 

 even when there is no mechanical interference with its progress. 



5. When a solid reacts with a dissolved reagent and the 

 reaction velocity at the boundary surface is limited, a balance 

 is quickly established between the consumption and the supply 

 (by diffusion) of the reagent, such that its concentration at the 

 boundary, under otherwise constant conditions, is always pro- 

 portional to its concentration in the solution. 



6. The rate of the reaction at the boundary surface may in 

 some cases be low enough, compared with the rate of the diffu- 

 sion process, to be an important, or even the predominant fac- 

 tor in determining the observed reaction velocity. A sound 

 interpretation of the diffusion theory must take account of this 

 possibility, which has heretofore been neglected. 



Art. XXXY. — Sulphatic Cancrinite from Colorado; by 

 Espek S. Larsen and George Steiger,* U. S. Geological 

 Survey. 



A number of specimens of the uncompahgrite,f collected 

 by the author (E. S. L.) from Beaver Creek, a tributary of 

 Cebolla Creek, on the Uncompahgre quadrangle, Gunnison 

 County, Colorado,^: showed a small amount of a secondary 

 mineral which could not be determined optically. A single 

 specimen from the northeast slope, about 50 feet below the 

 crest, of the hill that is surrounded by contour 8,500, and is 

 about half a mile southeast of the mouth of Beaver Creek, was 

 made up largely of a coarse-grained aggregate of this mineral. 

 From this specimen material suitable for chemical and optical 

 study was obtained and the mineral proved to be a cancrinite 

 in which nearly half the C0 3 was replaced by SO,,. 



In the field this specimen was thought to be an altered un- 

 compahgrite and the microscopic study confirms this. The 

 specimen is made up in large part of the sulphatic cancrinite 

 apparently derived from the original melilite, and contains also 

 considerable apatite, perofskite, and perhaps melilite. 



* Published with the permission of the Director of the United States 

 Geological Survey. 



f Laisen and Hunter, Journal Washington Academy of Sciences, vol. iv, 

 p. 473, 1914. 



tlhis work was carried on as a part of the aTeal mapping of the Uncom- 

 pahgre quadrangle, under the direction of Dr. Whitman Cross. 



