298 SCIENCE PROGRESS. 



is usually found in portions of the " country" rocks caught 

 up as inclusions by intruded magmas, and the reason of this 

 is easily understood. Moreover, in these included patches, 

 if anywhere, we may look for evidence of actual permeation 

 by the igneous magma, and consequent change in the bulk- 

 composition of the fragments affected. Unfortunately in 

 such cases we cannot always have certain knowledge of the 

 original nature of the included rock before its transforma- 

 tion. Among a number of descriptions of such inclusions 

 we may note that given by K. Vogelsang of the curious 

 patches enclosed in the andesite of the Bocksberg and of 

 Rengersfeld in the Upper Eifel (28). The constituent 

 minerals are cordierite in polysynthetic crystals, andalusite 

 in good prisms, sillimanite needles, felspar (mostly tri- 

 clinic) in very variable amount, flakes of biotite, octahedra 

 of green pleonaste, tabular crystals of corundum, etc. Very 

 various associations of these minerals occur, and the rela- 

 tion of the inclusion to the matrix also varies. The general 

 resemblance of these inclusions to what has been observed 

 in the thermal metamorphism of some crystalline schists is 

 evident, the abundance of aluminous minerals being again 

 very striking. For instance, one inclusion, composed largely 

 of sillimanite, gave about 47^ per cent, of alumina. Pre- 

 cisely similar inclusions occur in the andesite of the 

 Wolkenburg and the trachyte of the Drachenfels in the 

 Siebengebirge, while some of the volcanic ejectamenta of 

 the Laacher See also present many points of resemblance. 

 Vogelsang gives reason for regarding them all as fragments 

 of crystalline schists caught up in the igneous magma, 

 which has partially melted them and given rise to new 

 minerals. Some of the cordierite and the pleonaste are 

 certainly newly formed, and the author has artificially 

 imitated the phenomena by placing a fragment of cordierite- 

 gneiss in melted andesite. 



By a careful study of the inclusions in certain Scandi- 

 navian diabases, Backstrom (29) has succeeded in making 

 out something of the manner in which they have been 

 modified by the enveloping magma. This case differs, 

 however, from the others mentioned above in that the 



