PRAGMENTS OP OTHER ROCKS CONTAINED IN GRANITE. 



13 



of felspar usually show twinuing ; but the largest of them do not 

 exceed '2 millim. in length, while the flakes of mica seldom exceed 

 •15 millim. in diameter. In addition to felspar and mica this rock 

 contains a few minute grains of magnetite. 



The granites in the neighbourhood of Peterhead differ from those 

 in the vicinity of Aberdeen in their dominant felspar being of a 

 pinkish-red hue, instead of colourless or greyish-white, as in the last- 

 named locality. These granites are regarded by Professor Haughton 

 as being of eruptive origin, and their second felspar is albite*. 



Under the microscope thin sections of the granite from this 

 district are seen to consist of quartz, orthoclase, a subordinate amount 

 of a triclinic felspar, black or very dark-brown mica, a few needle- 

 like crystals of apatite, and occasional crystalline patches of titanite. 

 In addition to these minerals, vermicular chlorite occurs in the 

 granite of Black Hills, where the quartz also occasionally encloses 

 filamentary crystals, which are perhaps rutile. 



The principal quarries worked in the district are situated at 

 Stirling Hill and Black Hills, a few miles south of the town of 

 Peterhead. In both these localities the inclusions almost invariably 

 assume an ovoid form, and usually vary in dimensions from the size 

 of a nut to that of a large apple ; considerably larger ones are, how- 

 ever, sometimes met with. They are in nearly all cases darker in 

 colour and much finer in texture than the enclosing rock; their out- 

 lines are sharply defined, and they frequently enclose, porphyriti- 

 cally, rounded crystals of pinkish-red orthoclase. 



Seven different sections were prepared of as many ovoid inclusions 

 from this district, and in every instance they were found to consist 

 of an exceedingly fine-grained granite containing all the various 

 minerals present in the enclosing rock. The proportions of mica 

 and of triclinic felspar are, however, greater than in the normal 

 granite, and the amount of quartz is somewhat less. 



This agrees with the results obtained by analyses of the average 

 granite and of the dark-coloured rounded enclosures : — 



Granite. Inclusion. 



Wflter Jliygrometric '21 -19 



^ ^^^^ \ combined -40 '76 



Silica 73-70 64-39 



Alumina 14-44 15-99 



Phosphoric anhydride trace trace 



Titanic „ distinct traces distinct traces 



Ferric oxide -43 1-47 



Ferrous „ 1-49 5-98 



Manganous oxide trace trace 



Lime... 1-08 2-57 



Magnesia trace 1*67 



Potassa 4-43 2-46 



Soda 4-21 4-96 



100-39 100-44 



Specific gravity 2-G9 2-73 



* Proceedings of the Boyal Society, vol. xviii. p. 313. 



