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orthoclase W and sometimes the orthoclase is seen to be surrounded by a zone 

 of plagioclase. A polished slab illustrating this zoning of orthoclase by 

 plagioclase is preserved in the British Museum. Both the orthoclase and 

 plagioclase have been analysed by Dr. HEDDLE. (I) 



We have now referred at sufficient length to what may be termed the 

 normal plagioclase-rocks from which nepheline and leucite are absent. The 

 somewhat abnormal rocks generally known as the mica-traps will be 

 considered by themselves. 



(1) Trana. Roy. Soc. Edin. XXVIII., 1879, p. 2. 



