FRAGMENTS OF OTHER ROCKS CONTAINED IN GRANITE. 



17 



darker in colour than that at a greater distance from it, but does 

 not otherwise differ, excepting that fragments of felspar are some- 

 what more numerous, and that the mica has become almost entirely 

 replaced by green hornblende. 



Some of the inclusions found in this locality are unquestionably 

 fragments of a rock of the character of that exposed in the cutting 

 opposite the railway-station. These, when of moderate dimensions, 

 are generally almost black ; and when thin sections of them are ex- 

 amined under the microscope they are found to consist of granular 

 quartz with a few fragments of felspar, dark mica, green hornblende, 

 often some minute garnets, crystalline patches of sphene, and occa- 

 sionally a little pyrites. 



When included fragments of this quartzose rock are of large sizo 

 they, in the majority of cases, do not undergo alteration throughout, 

 but are darkened by the presence of a disseminated material of a 

 nearly black colour to a depth of about an inch only. In such cases 

 the junction of the inclusion with the surrounding granite is perfectly 

 defined, and the union of the two rocks is complete ; but the disse- 

 minated dark flakes gradually become less plentiful, and towards the 

 centre of the mass almost entirely disappear. 



On examining thin sections of the interior or central portions 

 of such inclusions they are fouud to be composed of a rock contain- 

 ing granular quartz, some felspar, numerous nearly colourless garnets, 

 a little dark mica, a few flakes of hornblende, and occasionally specks 

 of pyrites. 



The exterior or altered portions are found to contain the same 

 granular quartz mixed with a few fragments of felspar, much green 

 hornblende porphyritically enclosing grains of quartz, some dark 

 mica, a few small garnets, an occasional small crystal of sphene, 

 and sometimes a little iron pyrites or magnetite. 



The following analyses show the composition of the unaltered and 

 altered rocks respectively : — 



Unaltered rock. Altered rock. 



Water J ^g^-ometric -13 -08 



^^^^^ t combined -38 -36 



Silica 63-51 66-64 



Alumina 13-42 13-77 



Phosphoric anhydride trace trace 



Titanic „ traces traces 



Ferric oxide 1-14 s^ 2-15 



Ferrous „ 3-91 3-68 



Manganous oxide trace trace 



Lime 10-80 6-08 



Magnesia 3-03 3-42 



Sulphur trace trace 



Potassa -23 -40 



Soda . . 3-54 3-39 



100-09 99-97 



Specific gravity 2-84 2-79 



Q. J. G. S. No. 141. c 



