514 FREDERIC H. LAHEE 



each rock. They may occur as a relatively fine groundmass in 

 which ilmenite, biotite, garnet, and ottrelite may be present as 

 metacrysts. 



2. Minerals that crystallize or recrystallize with parallel 

 orientation promote the accommodation of the rock to the stress.^ 



3. A mineral that is shown to acquire parallel orientation at an 

 early stage of dynamic metamorphism will grow with parallel 

 orientation in later stages, provided shearing continues. 



4. A mineral that acquires parallel arrangement at a late stage 

 of dynamic metamorphism may develop at an earlier stage, but in 

 this case it originates after shearing has ceased and then has no 

 definite orientation. 



5. In the Narragansett Basin schists sericite is the first meta- 

 morphic mineral to appear and is the first to acquire definite 

 orientation of its crystals under conditions of stress. In Stage D it 

 may give place to muscovite. 



6. Quartz is chiefly clastic in the early stages of metamorphism 

 and secondary (recrystallized) in the later stages. In the accommo- 

 dation of the rock to stress it assists first by granulation and later 

 by recrystallization. It acquires dimensional, but not crystallo- 

 graphic, parallelism in Stages C and D. 



7. The order in which the minerals acquire both dimensional 

 and crystallographic parallelism, beginning with the earhest, is as 

 follows: sericite, ilmenite, biotite, ottrelite. Secondary quartz, 

 having only dimensional parallelism, would come between ilmenite 

 and biotite.^ 



8. The order of origin of the metacrysts, as shown by their rela- 

 tions to one another, is: ilmenite (first), garnet, biotite, and 

 ottrelite. Grubenmann calls this a crystalloblastic order (Reihe).^ 

 He gives the following succession for minerals of metamorphic 

 origin: titanite, rutile, hematite, ilmenite, garnet, tourmahne, 



' Cf. Leith: "Minerals showing the best evidence of recrystallization are those 

 best adapted by their shape and dimensions to conditions of unequal pressure," op. 

 cit., p. 95. 



= Leith places quartz after the micas in respect to its cleavage-making capacity. 

 Cemmencing with the best cleavage-maker, his order is: micas, hornblende, quartz, 

 and feldspar {op. cit., p. 64). 



3 Die kristallinen Schiefer, p. 91. 



