20 J. A. PHILLIPS ON CONCRETIONAKT PATCHES AND 



mica or dark hornblende in such bodies often causes them to be of 

 a deeper colour than the surrounding rock *. 



In addition to their general similarity in mineral constitution to 

 the adjoining granite, further evidence of the concretionary origin 

 of such inclusions is afforded by the fact that a second similar 

 nodule, differing from the first in colour or in fineness of grain only, 

 is occasionally found within them. 



Crystalline granitic inclusions not unfrequently assume the form 

 of imperfectly defined crystals of felspar ; these may be regarded as 

 an extreme development of the well-known phenomenon of the 

 enclosure of quartz and mica in felspar. 



Another form of inclusion, giving rise to the production of a dark 

 fissile patch in granite, results from a segregation of mica from the 

 surrounding rock. An example of this kind of inclusion is seen in 

 the granite from Kemnay, Scotland, fig. 6, PL I., where it will 

 be observed that in the immediate vicinity of the enclosure the rock 

 becomes comparatively free from mica. 



The formation of rounded inclusions in granite is believed to have 

 been usually contemporaneous with the solidification of the general 

 rock-mass, and to be due to forces of the same nature as those 

 which have resulted in the production of nodules in the well-known 

 Napoleonite or orbicular diorite of Corsica. It is, however, probable 

 that inclusions having a similar form may have been sometimes 

 produced by the enclosure of pebbles of extraneous rocks. 



In a few instances only have well-defined angular inclusions of a 

 fine-grained granite been observed in granites of a coarser descrip- 

 tion ; in some cases these may be enclosed fragments of an older 

 granite, but in all the specimens which have been examined, such 

 inclusions have invariably exhibited the distinctive characteristics 

 of the enclosing rock. 



Inclusions belonging to the second class are often schistose in 

 structure and are usually irregular in outline, being sometimes 

 traversed by veins of quartz, or divided by strings of granite, which 

 appear to have filled spaces caused by the floating apart of slaty 

 fragments in a granitic magma. 



In many instances they are not materially altered, and in a 

 majority of cases inclusions belonging to this class are easily re- 

 cognized as being fragments of either gneiss, mica-schist, hornblende- 

 schist, or garnet-schist ; comparatively few specimens only have 

 been met with so modified as to render their derivation difiicult to 

 determine. 



Slaty inclusions are, as a rule, either highly micaceous or highly 

 hornblendic ; and an examination of the schistose rock in contact 

 with the granite at Goragh Wood, and of partially altered inclusions 

 from the same locality, shows that in that case the production of 

 those minerals is a result of granitic contact t. It will be observed, 

 * When sections made throtigh both a pebble-like inclusion and the en- 

 closing granite are examined along their line of contact, minute crystals are 

 observed to extend from the former into the latter. 



t Mr. S. AUport has observed that in many cases slates in the immediate 

 vicinity of the Land's-End mass of granite have become echorlaceous (Quart. 

 Journ. Geol. Soc. vol. xxxii. p. 408). 



