G. F. Kurtz — Sapphires from Montana. 419 



logical Survey, in a chapter on The Production of Precious 

 Stones, — especially in No. 17, for 1895, p. 909 ; they will be 

 further described in No. 18, for 1896. 



Other localities are also coming to light in the same State ; 

 one of these is at Rock Creek, Granite County, thirty miles 

 from Phillipsburg, where the gems are reported of good blue 

 color, with other tints, and some pale rubies ; another is on 

 Cottonwood Creek, eighteen miles from Deer Lodge, — the 

 stones being of varied colors, red, pink, yellow, and occasion- 

 ally blue ; the third has been recently announced in Choteau 

 County. 



As regards the gems themselves, marked differences appear 

 in those from the two principal Montana regions. All are of 

 small size, but they differ in crystallization. Those from the 

 Missouri gravels are characterized by the presence of the pris- 

 matic faces, with the basal plane, and rarely any of the rhoui- 

 bohedral modifications, — the prevailing forms being hexagonal, 

 either prismatic or so short as to be tabular. A beautiful 

 example of this type is figured in " Gems and Precious Stones 

 of North America" (colored Plate I, fig. C). The specimens 

 from the minor localities have generally a similar type of form. 

 The Yogo Gulch crystals, on the other hand, are largely rhom- 

 bohedral, with the basal plane more or less present, but the 

 prismatic and pyramidal faces hardly at all. The rhombohe- 

 dron a?, which is prominent in these crystals, as shown in the 

 figures and descriptions of Dr. J. H. Pratt, has the remarkable 

 interest of being new to this species. Other very noticeable 

 features which the writer was the first to observe and point 

 out, are the striations on the basal plane parallel to its intersec- 

 tions with the rhombohedron, and sometimes rising into steps 

 as the oscillation becomes a replacement, as well shown in Dr. 

 Pratt's figures (Figs. lla-14a, p. 427), and the singular depres- 

 sions on the basal plane in other crystals l^Figs. 5-10, p. 425), 

 their sides being formed by faces of the inverse rhombohe- 

 dron, sometimes meeting in a point, and at other times trun- 

 cated and floored by a basal plane. 



We have here two distinct types of crystallization in the 

 same mineral, from the same State, and produced apparently 

 under similar conditions in igneous rocks. It will be extremely 

 interesting to learn, by further exploitation and study, whether 

 these two types bear any fixed or definite relation to the par- 

 ticular variety of eruptive rock in which they occur. The 

 accounts thus far given of the rocks examined seem to suggest 

 such a possibility. 



As to the value of the early Montana sapphires in jewelry, 

 it is hardly possible yet to predict how far it may be really 

 important. Much beautiful material has already been obtained, 



