242 DR. H. H. THOMAS OX XEXOLITHIC [vol. Ixxviii, 



In small xenoliths the buchite may be entirely mirepresented, and 

 the whole of the inclusion may then be composed of plagioclase 

 with accompanying corundum and spinel showing a small amount 

 of interstitial glass. 



I shall now briefly describe the chief minerals that enter into 

 the composition of the aluminous xenoliths, and at the same time 

 discuss in turn their probable mode of origin and relation to the 

 other minerals with which they are associated. 



(ii) The Minerals of the Xenoliths: their Mode 

 of Formation and Mutual Relations. 



Corundum. — The corundum of the xenoliths nearly always 

 occurs as deep-blue brilliant crj^stals of distinctly tabular habit. 

 There are three modes of occun-ence : — 



(1) As isolated crystals in the sillimanite-bucliites ; (2) in close association 

 with anorthite in the crystalline outer portions of the xenolithic masses 

 (PL YIII. fig". 2); and (3) as well-formed crystals in a matrix of distinctly 

 igneous natiire, which adheres to, and penetrates, the crystalline outer zone 

 of certain xenoliths (PL IX, fig. 1). 



All crystals have the same general habit; the}^ are flattened 

 parallel to the base {0001}-, and are usuall}" rhombohedral or 

 pyi-amidal in form. The prism of the second order -111201 is 

 seldom met with, and the usual combination of forms is the base 

 {0001} modified by the rhombohedra {1011[ {3032[, or less 

 commonly by the rhombohedra together with the p3a'amid {2248}. 



The faces are bright, and give excellent goniometric readings, 

 b}^ which the identit}^ of the forms mentioned above has been 

 established. The basal plane is stepped or deeply striated parallel 

 to the trace of one or other of the rhombohedral faces, giving rise 

 to Avell-marked equilateral triangular areas, one within the other, 

 which are due to oscillation in regrowth of the base and rhombo- 

 hedron. In size the crystals vary from a millimetre or so, 

 measui'ed across the basal plane, to at least a centimetre and a half 

 in exceptional cases. Their thickness measured along the vertical 

 axis is usually less than half the width of the basal plane. The 

 acutely pyramidal habit of many varieties of corundum is entirely 

 imrepresented. The tabular habit and general simplicity of forms 

 appear to characterize corundum of xenolithic masses, and, what 

 practicall}^ amounts to the same thing, that which has separated 

 from basic igneous magmas on their becoming saturated with 

 alumina through solution of aluminous material. 



Corundum such as Ave are considering has been fully and 

 repeatedly described, and it will be suflicient to state that the 

 crA^stals here mentioned find their closest analogue in the tabular 

 crystals of Yogo Grulch (Montana).^ 



^ For detailed accounts of the occiu-rence of this mineral, see J. H. Pratt & 

 J. V. Lewis, ' Corundum & the Peridotites of Western Xorth Carolina ' Mem. 

 Geol. SuTV. N. Carolina, vol. i (1905) pp. 225 et seqq.\ L. Y. Pirsson, Amer. 

 Jour. Sci. ser. 4, yoL iv (1897) p. 421 ; also 20th Ann.' Eep. U.S. Geol. Surv, 

 1898-99, pt. iii (1900) pp. 552, 553. 



