170 



PETROLOGICAL NOTES ON SOME OF THE ERRATICS 



Microscopic Characters. — The rock is hypidiomorphic granular in fabric 

 with a medium grainsize. Felspar makes up about 80 per cent, of the minerals 

 present. 



Plagioclase about Ab 5 An 3 , is present in subidiomorphic sections up to 1 mm. dia- 

 meter. It shows twinning after the albite Carlsbad and Manebach laws, the striations 

 being very fine and showing pretty examples of bending. The mineral is somewhat 

 decomposed, particularly near the periphery. 



Orthoclase is untwinned, slightly decomposed, and exhibits shadowy extinction. 

 It is moulded on plagioclase, and in some cases there is apparently a crystallographic 

 relation between the two, the outgrowths of orthoclase showing optical continuity 



Quartz is not very allotriomorphic, and shows shadowy extinction due to strain. 

 Biotite calls for no special description, it is somewhat frayed and contains inclusions 

 of apatite. 



There is also a very little light-coloured epidote. 



Fig. 1. Sketch showing Kelation of Plagioclase and Orthoclase. The grains of 

 orthoclase indicated by X are optically continuous. 



439. Aplite 



Microscopic Characters. — Allotriomorphic granular rock with an average 

 grainsize of '5 mm., consists essentially of quartz and orthoclase with less abundant 

 albite and very little decomposed biotite. 



Quartz shows slight evidences of strain ; orthoclase is quite ordinary. The 

 plagioclase present is between albite and oligoclase in composition. 



Rocks of this type are extremely abundant amongst the erratics ; a fact attribu- 

 table to their resistance to weathering being greater, as a rule, than that of the normal 

 biotite or hornblende granites with which they were in all probability genetically 

 connected. 



