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what may be termed the ideal condition. Two or more of the distinguishing 

 minerals are usually found occurring together in the same rock. Thus we 

 have muscovite-biotite-granites ; biotite-hornblende-granites, and biotite- 

 augite-granites. Augite-bearing granites are comparatively rare. 



Granitite. This name was proposed by G. ROSE for a granite containing 

 much oligoclase and a black mica, but no original white mica. It is so used 

 by most German authors. 



'Granulite. French authors use this term for an eruptive granite 

 containing both micas. Thus the Cornish and Aberdeen granites would be 

 termed granulite by French petrographers. German authors use the same 

 name for a rock of doubtful origin in which garnet is frequently present as 

 an accessory constituent. 



^Aplite. This name has been applied to a rock composed almost 

 entirely of quartz and felspar. ROSENBUSOH proposes that it should be 

 used so as to include the muscovite-granites. Aplite or muscovite-granite 

 is not known to form independent rock masses of considerable extent. It 

 occurs rather as veins and dykes, or as the apophyses of ordinary 

 granites. 



Pegmatite. This_name was applied by HAUY to the definite intergrowth 

 of quartz and felspar otherwise known as graphic granite. NAUMANN 

 altered the signification of the term and applied it to extremely coarse 

 aggregates of quartz, orthoclase and mica. Pegmatite (H.AUY) and 

 pegmatite (NAUMANN) are often found together. They occur rather as 

 segregations than AS independent masses of eruptive origin. 



Felsite, Eurite, PetrosUex. These terms are practically synonymous. 

 They have been applied to compact, stony rocks the mineralogical 

 composition of which cannot be ascertained by examination with the naked 

 eye or with a lens. These rocks are anhydrous (or nearly so) and except in 

 this respect agree in composition with the acid glassy lavas. The first term 

 was proposed by GERHARD (1814), the second by DAUBISSON (1819), the 

 third by BROGNIART. We shall use the first term only. Many of the 

 felsites have been shown by RUTLEY and BONNEY to be devitrified obsidians 

 and pitchstones. ^ Such i felsites represent the glassy lavas of the early 

 geological periods. 



Qtiartz-felsite. This term has come into extensive use in this country 

 for a rock containing porphyritic crystals of quartz and felspar in a matrix 

 of felsite. 



Quartz-porphyry. This term is used on the Continent for pre-Tertiary 

 porphyritic rocks of acid composition. It includes the quartz-felsite of 

 British petrographers and also the porphyritic pitchstones and obsidians of 

 pre-Tertiary age. 



Fdsite-porphyry. This term is also extensively used on the Continent 

 for any porphyritic rock with a felsitic ground-mass. It includes the quartz- 

 felsites and also rocks that may be . termed ortho-felsites (orthophyres of 

 some authors) that is rocks containing porphyritic orthoclase without 

 porphyritic quartz. 



Elvanite. This is derived from a Cornish miners' term It includes 



