S. L. Penjield — Beryllium Minerals from Colorado. 491 



matter of interest. Mr. Geo. L. English of Philadelphia 

 loaned nie from his private collection a beautiful phenacite 

 crystal, fig. 2, which had partially grown over and inclosed a 

 bertrandite and beryl crystal, showing that in this case at least 

 phenacite is a younger mineral than bertrandite. 



The author would also acknowledge two corrections which 

 should be made in the first of the above mentioned bertrandite 

 papers. One, pointed out correctly by Carl Yrba," is that 

 the observed twinning plane is the unit brachydome l-l, Oil 

 instead of the steeper S-l, 031. This mistake must be attributed 

 to carelessness on the author's part as seen by reference to his 

 note book. The other is a mistake in the calculation of the 

 vertical axis caused by an error in copying one of the measured 

 angles. The length of the vertical axis should be 0*5993 in- 

 stead of 0*5953 as pointed out by C. Hintze.f 



3. Phenacite. 



In the present article the author desires to call attention to 



the very beautiful crystal belonging to Mr. English, which was 



just mentioned, in which a rather unusual habit is derived, fig. 



2, from the almost equal development of the faces at the ends 



of the crystal, the forms being the same as those already 

 identified and figured, *j: m, 1010, I; a, 11 20, i-2 ; r, 1011, f 1; 

 x, 1322, -rf-f ; s, 2131, +r 3-f and d, 0112, -i A number of 

 twin crystals have also been observed in which the base can be 

 taken as the twinning plane and the two crystals, with parallel 

 axes, show reentrant angles only between the rhombohedrons 

 at the ends, fig. 3. Some of these are of almost ideal 

 symmetry. 



Mineralogical Laboratory of the Sheffield Scientific 

 School, New Haven, March, 1890. 



*Zeitschr. Kryst., xv, p. 470. f Handb. der Mineralogie, p. 413, 18yu. 



% This Journal, xxxiii, 1887, p. 133, and xxxvi, 1888, p. 321. 



