532 AUSTIN F. ROGERS 



of fossilized bones consisting of collophane which shows an appre- 

 ciable amount of double refraction. The double refraction is 

 variable from spot to spot, and the result is a characteristic wavy 

 extinction. 



In view of these facts the safest method of distinguishing collo- 

 phane from dahllite is usually by means of the index of refraction. 

 I have determined the index of refraction of collophane from 

 many localities and have found that it usually varies from 1.57 to 

 about 1 . 6 1, 1 while that of dahllite varies from about 1 . 61 to 1 . 63. 

 With high magnification dahllite shows a fabrous structure which 

 is lacking in collophane. 



Like dahllite, collophane is soluble in hot nitric acid with 

 effervescence, but in the closed tube it gives a great deal of water 

 while dahllite gives little or none. 



The quercyite of Lacroiz 2 is collophane and not a mixture of 

 dahllite and collophane. I have examined a specimen of a banded 

 calcium carbonophosphate mineral labeled " Hydroapatite, Mar- 

 seilles, France," which is identical with Lacroix's quercyite. The 

 doubly refracting layers of my specimen and probably of the 

 specimens figured by Lacroix (op. cit., p. 580) are collophane and 

 not dahllite. In support of this view it may be mentioned that 

 the index of refraction (1 . 598=!= o. 002 in my specimen, but Lacroix 

 gives 1 . 608) is too low and the water content (3 . 2 to 6 . o per cent) 

 too high for dahllite. Double refraction is by no means a proof of 

 crystallinity. Lacroix also mentions the fact that the fibers of 

 quercyite lack the individuality of those of dahllite (and staff elite), 

 which is a good argument in favor of its being collophane. 



Monite from Mona Island in the West Indies is also a variety of 

 collophane. It is a white, earthy, massive mineral which, under 

 the microscope, is largely isotropic with small doubly refracting 

 spots. The index of refraction is 1.631=1=0.001, a little higher 

 than for most specimens of collophane. Monite effervesces vig- 

 orously in hot nitric acid, so that the carbonate radical was over- 

 looked in Shepard's analysis. In the closed tube it gives abundant 



1 The fluorcollophanite described by Artini (loc. cit.) has an index of refraction of 

 1.630. 



3 Mineralogie de la France, IV (1910), 579. 



