28 Kraus and Cook — Datolitefrom Westfield, Mass. 



This similarity in the composition of the datolites from these 

 two localities becomes more pronounced when we consider that 

 the value given by Bodewig for boron trioxide is the mean of 

 three determinations, one of which is 21*6 per cent. This 

 agrees very closely with the values obtained by us. 



Specific Gravity. — For the determination of the specific 

 gravity four clear crystals of convenient size were used. The 

 determinations were made by means of the hydrostatic balance 

 at a room temperature of 19*5° C, the water being 21 '5° C, 

 with the following results : 



I 3 -0005 



II 2-9998 



III 3-0065 



IV .3-0165 



Average 3-0058 



The values for the specific gravity are usually given in the 

 various standard text-books* as varying from 2*9 to 3. Bauer, f 

 however, gives 2 # 9-3'6. We have been unable to find any 

 records whatever of datolites possessing a specific gravity as 

 high as 3*6, and on account of the fact that the figures obtained 

 by us on very pure material are much lower, we would question 

 the correctness of the larger value given by Bauer. 



In conclusion, we wish to express our indebtedness to Prof. 

 E. D. Campbell, Director of the Chemical Laboratory of this 

 University, for advice and suggestions relating to the chemical 

 portion of this paper. 



Mineralogical Laboratory, University 

 of Michigan, Ann Arbor, Mich. 

 March 30, 1906. 



* Dana, System of Mineralogy, 1892, 504 ; Hintze, Handbuch der Mineral- 

 ogie, II, 167 ; Miers, Mineralogy, 1902, 588 ; Naumann-Zirkel, Elemente der 

 Mineralogie, 14te Auflage 1901, 629. 



f Bauer, Lehrbuch der Mineralogie, 2te Auflage, 1904, 762. 



