OCCLUSION OF IGNEOUS ROCK 889 



even to total absorption of such enclosures — or, as it has been 

 well expressed, " the striking results of extreme contact meta- 

 morphism, displayed by fragments of rocks which have lain for 

 a time in the bath of a molten igneous magma."^ 



In the intimate association of widely diverse types of igneous 

 rocks in the Tertiary of Great Britain similar results to those 

 already noted on Manhattan Island have been recently de- 

 scribed — "the tendency of an acid intrusion to follow closely 

 the line of an earlier basic intrusion." ^ 



But, in the igneous rocks of Skye, this has resulted, in different 

 cases, in production of " a rock with evident xenoliths, more or 

 less altered, a hybrid product with scattered xenocrysts, usually 

 much disguised, or, in the extreme case, a rock which shows in 

 a given specimen no direct indication of any foreign element. 

 Even this last, however, will often betray its origin by some- 

 thing unusual in its mineralogical constitution." ^ The maxi- 

 mum effects were found where a basic rock had been attacked 

 by an acid magma, especially under one favorable condition, 

 previously pointed out,^ which has ordinarily prevailed during 

 penetration of a solid body of igneous rock by a fluid magma, 

 that " reactions . . . will be promoted by the former being 

 still at a high temperature when the latter comes into contact 

 with it." 



The same observer has thus accounted also " for the con- 

 tinuous or lenticular bands of basic rocks which are in places 

 associated with the more acid and hybrid rocks as integral parts 

 of the complex. These basic rocks appear to have been of the 

 nature of gabbros, now transformed by metamorphism, and in 

 some measure by interchange of material with the acid magma. 

 A dark hornblendic rock of this kind, with more or less evident 

 banding and foliation . . . has the general aspect of a medium 

 grained diorite. In a thin sHce it is seen that the deep green 



1 J. W. Judd, Quart. Jour. Geol. Soc, Vol. XLIX, 1893, P- 175- 



2 For instance, the pegmatite dike which can now be seen following the foliation 

 of diorite-schist down the bluff at West 130th Street and St. Nicholas Avenue, Man- 

 hattan Island. 



^A. Harker, Quart. Jour. Geol. Soc, LIX, 1903, 210-212. 

 * Idem., Jour. Geol., VIII, 1900, 394. 



