THE 



QUARTERLY JOURNAL 



OE 



THE GEOLOGICAL SOCIETY OF LONDON. 



Vol. XXXVI. 



1. On Concretionary Patches and Fragments of other Kocks con- 

 tained in Granite*. By J. Arthur Phillips, Esq., F.G.S. 

 (Eead November 19, 1879.) 



[Plate I.] 



!No one who is in the habit of visiting granite-quarries, or who 

 has had frequent opportunities of examining granite, either in a 

 dressed or in a polished state, can have failed to observe that it 

 frequently contains patches which resemble imbedded fragments of 

 older rock. These patches or nests are usually, although not always, 

 darker in colour than the granites in which they occur, while their 

 form may be either rounded or more or less angular. In the first 

 case the patch resembles an enclosed pebble, while in the second it 

 often presents the appearance of a fragment of slate or mica-schist. 

 Such patches most frequently have their outlines clearly and 

 sharply defined, but they occasionally merge by almost insensible 

 gradations into the enclosing granite. 



They are usually finer in grain than the granites in which they 

 are found, and, from being less easily attacked by atmospheric 

 agencies, not uncommonly stand out in considerable relief from the 

 surfaces of weather-worn boulders. The union of the enclosed 

 p«tch with the enclosing rock is generally complete, and their 



* In this paper fine-grained granitic patches or nodules, although not exhi- 

 biting a concentric structure, are spoken of as concretionary. They are also 

 fcometimes mentioned as concretions. 



Q. J. G. S. No. 141. B 



