Cook, and Kraus — Datolite from Great Notch. 645 



Selective Coating. — The examination of a large number of 

 the crystals shows that certain faces are always dull. This 

 dullness was at first believed to be due to natural etching. 

 However, it was found that when the crystals were boiled in 

 dilute hydrochloric acid and then suddenly cooled, dull appear- 

 ing material separated off in the form of flakes leaving a shiny 

 surface, which generally showed some signs of natural etching. 

 As shown in fig. 2, the pinacoid a jlOOf and the pyramid 

 n\\\\\ are always free from this coating, while the same is 

 generally true for e jH2j. The other prominent faces at least 

 are coated. (The crystal is represented in fig. 1 without the 

 coatings.) Generally the coating is much heavier on the ortho- 

 dome a? j 102 J than on the other forms. The prism mjllOJ 

 has the next heaviest coating and the clinodomes have the 

 lightest. Another interesting feature of this coating is the 

 occurrence of a beveling on the lower edge of the coating of 

 the orthodome x |102J upon which appear striations that are 

 apparently parallel to the plane of symmetry of the crystal. 

 An examination of the distribution of the coatings shows that 

 they are symmetrical to the plane of the a and e axes, to an 

 axis of binary symmetry parallel to the b axis, and to the cen- 

 ter, thereby placing the crystals in the prismatic class of the 

 monoclinic system to which they have been assigned upon mor- 

 phological grounds. 



With regard to the composition of the coating it may be 

 pointed out that some of the specimens show stilbite in asso- 

 ciation with the datolite in such a relationship as to leave no 

 doubt that the stilbite is younger. This leads to the conclu- 

 sion that the coating on the datolite is stilbite. This hypo- 

 thesis is also supported by the fact that in some instances the 

 coating presents a sheaf -like appearance. 



University of Michigan, 

 Ann Arbor, Mich., 

 November 9, 1914. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIX, No. 234.— June, 1915. 

 42 



