240 DE. H. H. THOMAS 0>" XEXOLITHIC Vol. lxx\'iii, 



reverted to qiiartz. and usuallv extinguish simultaneously with the 

 quartz-grain that thev fringe ; but, recalling the identical stinc- 

 tures produced in fused ganisters and other siliceous refractories,^ 

 we can have no doubt as to their original character. 



Dr. J. S. Flett.- some rears a 20. in a xenoHth from Trais'h 

 Bhan na Sgun-a, described fiiuges identical with those mentioned 

 above, but doubtfully attributed to them a different origin. 



Of the recently collected mateiial one of the best examples of 

 such a siliceous xenolith comes fi'om Feorlein Cottage, Carsaig 

 (Locality 3). and is illustrated in PL YII. fig. 5. The quaitz has 

 suffered considerable solution by the interstitial melt ; but, on 

 cooling, large plates of tridymite have been deposited upon the 

 undissolved quartz, and similar crystals have separated from the 

 melt now represented by the glassy matrix. 



The amount of glass found in these siliceous xenoliths is variable, 

 and appears to increase according to the amount of felspathic 

 constituents in the original rock, also with transfusion of igneous 

 material which would tend to lower the melting-point. The 

 presence of igneous material usually betrays itself by the deeper 

 colour of the interstitial glass, and by the sepai-ation of minute 

 crystals of eordierite. A beautiful example of the ready fusion 

 of the felspathic constituents and the formation of a copious inter- 

 stitial melt has been furnished by a sandstone xenolith (20763) 

 eolleeted by Mr. E. M. Anderson from a tholeiite-siU nearBunessan. 

 A sandstone xenolith metamorphosed by a tholeiitic magma. 

 and modified to some extent by transfusion of igneous material. 

 is illusti-ated by an example from south-east of Gortein Driseach. 

 between Localities 2 & o PL VII. fig. 6). 



{b) The Aluminous Xeno'idis. 

 (i) General Description. 



The aluminous xenoliths present three distinct types, and may 

 be grouped under three separate headings, according to their 

 structure and mineral constituents, and incidentallv accordincf to 

 the manner and degree of modification induced by the igneous 

 magma. The following types, which occur either in close depen- 

 dent association or as separate unitS; can be readily recognized : — 



(1) The sUliinaiute- and cordierite-buchites. 



(2) The anorthite-eomiidttm-spiiiel assemblaffe. 



(3) The eordierite- sillimaiiite- spinel assemblag'e. 



(1) The buchites(or basalt-jaspers of foreign wiitersi are 

 composed essentially of glass with one or more characteristic 

 crystalline phases, such as sillimanite, eordierite, etc. They are 



1 See H. H. Thomas, A. F. Hallimond, i E. G. Eadley, ' ]Mineral Eesonrces 

 of Great Britain ' Mem. GeoL Snrv. toL xri (1920) p. 60 & pi. It. 



- ' Geology of Colonsav <fc Oronsay. ^th part of the Eoss of Mtdl ' Mem. 

 GeoL Snrv.^i911. x>. 9-5 & t-l. vi. fie. -5. 



