J. S. Diller — Mineralogical Notes. 217 



mineral is evidently rhombic* In cross sections imperfect 

 cleavage is rarely seen parallel to some prismatic plane. Poly- 

 synthetic twinning is very common parallel to b x as well as 

 other planes in the prism zone. Liquid inclusions and long 

 tubular cavities parallel to the vertical axis are abundant. 

 Hardness =7 and sp. gr. slightly above 3'265. 



The rock in which the dumortierite occurs at Harlem is the 

 pegmatoid portion of a biotite gneiss. These coarse vein-like 

 parts are composed of quartz with both red and colorless or- 

 thoclase, some plagioclase and tourmaline. The other portions 

 of the rock contain much biotite and garnet. The fibers of 

 dumortierite are sparingly scattered through the quartz in the 

 coarse granular rock. A few were observed penetrating plagi- 

 oclase. The thin thread-like fibers are occasionally so small as 

 not to be distinctly dichroic, but they are intermingled and 

 connected with larger dichroic fibers by every intermediate 

 gradation in size, so that an observer at once regards them all 

 as the same mineral. They sometimes closely resemble the 

 trichitic forms in granitic quartz which Dr. G. W. Hawesf and 

 many others following his suggestion, regarded as rutile. 



The presence of tourmaline in the rock at Harlem was not 

 at first recognized. It is so intimately associated with the du- 

 mortierite that they cannot be easily separated. Their pleo- 

 chroic phenomena, however, are so unlike that they can be 

 readily distinguished under a polarizing microscope. The 

 presence of tourmaline renders the results of Mr. Biggs' anal- 

 ysis less trustworthy. By means of the Klein solution and an 

 electro-magnet the tourmaline was separated from the dumorti- 

 erite. The *217 gram of the latter thus obtained was analyzed 

 with the following result.;}; Only the smallest trace of B 2 3 

 was observed. 



Si0 2 31-44 A1 2 68-91 



Fortunately, at the time it became particularly desirable to 

 obtain a larger quantity of dumortierite for analysis a collec- 

 tion of minerals was sent by Mrs. C. A. Bidwell from Clip, 

 Yuma Co., Arizona, to the National Museum for identification. 

 Among them Prof. F. W. Clarke noticed a blue mineral which 

 proved to be dumortierite. It is finely fibrous and so abundant 

 as to give color to the rock which is composed chiefly of gran- 

 ular quartz. A few grains of magnetite and limonite are the 

 only other minerals intermingled with the quartz and dumorti- 

 erite, so that it seems an easy matter by means of a heavy solu- 



* My observations, noted in Mr. Riggs's paper, already referred to, were very 

 hastily made with imperfect apparatus, and published before I had an opportu- 

 nity for their revision. — J. s diller. 



■f Mineralogy and Lithology of N. H., p. 45, 1878. 



% The chemical work for this paper was done by Mr. Whitfield. 



