422 Pirsson — Corundum-bearing Rock from Montana. 



These two minerals lie closely crowded together and no 

 feldspars are seen in the rock. The interstices between them 

 consist of a small amount of a clouded, brownish, kaolin-like 

 aggregate, which appears to represent some former feldspath- 

 oid component, possibly leucite, perhaps analcite. The rock 

 appears to have its closest affinities in the monchiquite group, 

 of which it may be considered a basic, somewhat altered type. 

 The abundance of biotite shows its relation to the minettes, 

 but the rock is much richer in the ferro-magnesian components 

 and lacks the feldspar of the minettes. It has evidently a close 

 affinity with the minettes and shonkinite of the region whose 

 occurrence has been already described,* and is clearly a more 

 basic form of the same magma. It has the same richness in 

 biotite and pyroxene as these, but differs in the feldspathic 

 component. The Yogo Peak center is but a small number of 

 miles distant from the locality. 



Some calcite in agglomerated granules is also seen in the 

 section and this, as is so often the case in lamprophyres, does 

 not appear as if secondary in origin and is probably due to 

 limestone fragments picked up as previously mentioned. 



Origin of the sapphires. The occurrence of such well- 

 crystallized corundum in a basic igneous rock is of great 

 interest. It seems clear, from the many different ways in 

 which this mineral occurs, that there must be several methods 

 in nature for its formation. The association with metamor- 

 phic rocks such as gneisses, schists, etc., is well known and its 

 occurrence with granites is also not uncommon. In all these 

 cases, however, the association is with older, metamorphic or 

 granular crystalline rocks, and we know of its occurrence in 

 more recent, undoubted, basic, igneous rocks in but few cases. 

 Lagorio,f in an article to be mentioned presently, gives a list 

 of the known occurrences of corundum in igneous rocks, their 

 tuffs, ejected fragments and contact zones. The number of 

 occurrences where the mineral is found imbedded in igneous 

 rocks is small, and to them the author can add Unkel on the 

 lihine and Steinheim near Frankfort on the Main, where, 

 as he has observed, small blue sapphires enclosed in the fresh 

 basalt have been found. 



By a series of important and interesting experiments Moro- 

 zewiczj showed that molten glass of a basic character dissolved 

 alumina readily and in large quantity, and from this, on cool- 

 ing, corundum and spinel crystals separated out. Lagorio,§ in 

 commenting on these results and adding details of some exper- 

 iments of his own, showed that the former idea which had 

 been held concerning the origin of corundum in igneous rocks 



* This Journal, vol. 1, 1895, p. 467. 



f Zeifcschr. far Kryst., vol. xxiv, p. 285, 1895. 



X Ibid., vol. xxiv. p. 281, 1895. § Op. cit., supra. 



