BY W. G. WOOLNOUGH. 805 



good deal of zircoa and apatite. The fenomagnesian minerals 

 are a good deal chloritized, but not exceptionally so. 



Quartz is thoroughly interstitial, and is optically continuous 

 over quite wide areas, so that the structure is markedly poikilitic. 



No. 9 is evideutly the same rock, intensely altered. The body 

 colour is pinkish-grey, from the abundance of pink aggregates of 

 secondary minerals. No quartz can be seen, and the ferromag- 

 iiesian material is almost entirely hornblendic. 



Under the microscope, the most conspicuous departure from 

 the unaltered rock is the entire absence of quartz. This may be 

 •due to one of three causes : zonal differentiation may have caused 

 •the formation of more basic minerals peripherally, the crystall- 

 izing mother-liquor containing the last constituents of the magma 

 to separate (including the quartz) escaped into and caused the 

 alteration of the adjacent limestones, or the magma picked up so 

 much lime from the sediments as to completely absorb all the 

 silica to build silicates. 



The minerals present are sufficiently like those of the unaltered 

 grano-diorite as to suggest close relationship. Felspar in abun- 

 dant but completely clouded for the most part, and converted 

 into milky pseudomorphs giving aggregate polarization. Much 

 of the secondary mineral is sericite. Some of the felspar is not 

 so completely altered as to destroy all traces of twinning; and, 

 though satisfactor}^ measurements cannot be obtained, the felspar 

 is seen to be a basic plagioclase. Hornblende is still recognizable, 

 but is in process of metasomatic alteration. It is much shredded 

 and bleached, and is partially replaced by a colourless pyroxene 

 developed in parallel orientation with it. Biotite is absent, but 

 is represented by nearly colourless chlorite in large flakes and 

 fibrous radial aggregates. Sphene is rather abundant in small 

 crystals and grains, whose distribution suggests that they have 

 been formed as a result of the disintegration of the biotite. 

 Only a little magnetite is visible. 



No.8 is probably the same rock as No. 9, with increased altera- 

 tion of the original constituents. Felspars are completely 

 sericitized, no unaltered ones remaining. Secondary mica has 



