274 Mineral Resembling Aleerschaum 



with the exception that the base (cx)i) is lacking and the 

 brachypinacoid ei' (014) replaces the form e* (oi2) and the 

 niacropinacoid i^^ (loi) is replaced by the form u (103). (See 

 plate vii, figs, iii, iv, v, and vi.) 



Through the courtesy of Prof. B. K. Emerson, the mate- 

 rial described and figured by himt has been at hand for com- 

 parison. (See plate vii, fig. vi.) In his description of the 

 material it is to be noted that he orients his crystal in the 

 same way as the present author, and although he mentions 

 five forms he only gives two measurements, one of which ap- 

 pears to be correct and the other incorrect, on account of an 

 imperfection in the crystal which would readily be overlooked 

 without other material at hand for comparison. From a care- 

 ful examination of Prof. Emerson's specimen, it appears al- 

 most impossible to secure measurements that will do more 

 than give an approximation for any faces except the prism 

 which he gives as Oop (no) and the brachydome which he 

 gives as 2P00 (021). In the case of prism my measurements 

 agree with those of Prof. Emerson, but in the case of the 

 brachydome, my measurements range from 92° to 95° while 

 he gives the angle as ranging from 79° to 81° 30'. This dis- 

 crepancy is due in all probability to the presence of a pseudo- 

 cleavage in the fibrous coating of one of the crystals, as by 

 measuring along this cleavage I secured approximately the 

 same angle as Prof. Emerson. 



By orienting the crystal so that 010 becomes 100 it is 

 possible to get all the prismatic and normal faces to correspond 

 to possible olivine forms but with two exceptions they have 

 not yet been found on olivine crystals and it seems unreason- 

 able to refer the source of this material to olivine when most 

 of the faces do not correspond to known forms of olivine. 

 These faces, do, however, closely approximate the forms of 

 humite, the association is like that of humite, and if any other 

 ground were necessary for discarding olivine as the source 

 and substituting humite, it would be found in the size of the 

 crystals which vary from about Va" to 2" in length, the largest 

 under investigation being the latter size, and the average 

 about i/'2". Inasmuch as an olivine crystal two inches long 

 is looked upon as of extraordinary magnitude, the presence of 



i-Emerson, B. K., U. S. Geol. Sur. Bull. 12 6. 1895, p. 92, pi. 1, fig. 4. 



