so. 2240. XOTfJS ON MIMETITE, ETC.— WHERRY. 375 



The results obtained were: Lead oxide 73.3, chlorine 2.5, arsenic 

 pentoxide by difference 24.7 per cent, agreeing closely with the theory 

 for mimetite. 



This constitutes a good example of the value of optical study of 

 rare or unsual minerals. Had Mr. Larsen not examined this speci- 

 men and discovered that its optical properties differed from those of 

 penfieldite, it would in all probability have continued indefinitely to 

 be treasured as a specimen of that rare mineral, which it certainly 

 resembles in superficial aspect more than it does mimetite. 



TIL\UMASITE. 



CBY ST ALLOGRAPH IC MEASUBEMENTS OF THAUMASITE. 



The first thaumasite discovered, from several localities in Sweden, 

 was massive, but proved to be optically uniaxial, showing it to belong 

 either to the tetragonal or hexagonal crystal system, and it is so 

 classed by Dana.^ 



The material subsequently found at West Paterson, New Jersey, 

 was described by Penfield and Pratt as forming a loose aggregate of 

 hexagonal prismatic crystals.^ A terminated crystal has been re- 

 cently measured by Dr. W. T. Schaller;^ it shows the base, 0001, a 

 pyramid p, taken as the unit, 1011, and a prism, m, of the same order 

 as the pyramid, the symbol of which is accordingly lOTl. The angle 

 p between the pyramid and the base averaged 51° 30', whence Doctor 

 Schaller calculated the axial ratio of the mineral to be (?=1.09. 



Early in 1916 Mr. James G. Manchester, president of the New 

 York Mineralogical Club, sent the United States National Museum 

 a number of minerals from New York and New Jersey in exchange, 

 and among the lot was 25 grams of crystallized thaumasite, repre- 

 senting about 50,000 tiny crystals, mostly less than 1 mm. in length. 



The vast majority of the crystals, though doubly terminated, show 

 but two forms, the first order prism and the base, but three hours' 

 search under a binocular microscope disclosed five crystals showing 

 several distinct pyramidal faces and a few faces of the second order 

 prism. These were submitted to crystallographic measurement, and a 

 preliminary announcement of the results was made in August, 1917.* 



Shortly after the appearance of this preliminary announcement 

 there was received in this country from Stockholm, SAveden, the April, 

 1917, number of the Geologiska Foreningens Forhandlingar, in which 

 Dr. Gust. Flink announced the discovery of measurable crystals of 



1 System of Mineralogy, ed. 6, 1S92, p. 698. 



- On the occurrence of thaumasite at West Paterson, N. J. Amer. Jour. Sci., ser. 4, 

 vol. 1, 1896, p. 229. 



» The crystallography of thaumasite ; in Mineralogical notes, Series 3 ; Bull. U. S. Geol. 

 Surv. 610, 1916, p. 130. 



*Amer. Mineralogist, vol. 2, 1917, p. 89. 



