F. H. Lahee — Metamorjphism and Geological Structure. 357 



Under the microscope the finer portions of the rock (ground- 

 mass when metacrysts occur) display certain features different 

 from those seen in Stage B. For instance, in addition to the 

 straining, crushing, and slicing of the quartz, some grains are 

 partly or wholly recrystallized (see fig. 11) ; that is, some of 

 the quartz is metamorphic. Furthermore, occasionally a little 

 secondary aibite or oligoclase may be discovered ; and the 

 ilmenite individuals have grown so large and conspicuous as 

 to be classed with the metacrysts. 



Fig. 1. Fig. 2. Fig. 3. 



t* 



Figs. 1-3. Progressive elongation of crystals of ilmenite accompanying 

 increase of metamorphism. All are of secondary origin. The outlines in 

 fig. 1 and fig. 2 and the white areas included in fig. 2 are due to quartz. The 

 arrows indicate the direction of the schistosity. 



The ' knots,' or ' knoten,' are oval granular aggregates (see 

 figs. 13 and 14), measuring one millimeter or less in length 

 and lying with their long axes parallel to the cleavage. They 

 consist of quartz, chlorite, muscovite, and calcite, with a little 

 ilmenite, rutile, and limonite. While generally lacking signs 

 of distortion, in zones of maximum shearing their constituents 

 have assumed positions of alignment parallel to the adjacent 

 groundmass minerals. The cleavage of the rock curves round 

 them. Laterally they are confined by strings of sericite ; at 

 their ends the sericite of the body of the rock gradually gives 

 place to chlorite and muscovite, micas which are especially 

 characteristic of these knots. 



Although any of the minerals just cited may be found in 

 any portion of a knot, there is ordinarily an obscure concentric 

 arrangement, particularly in the least sheared forms (fig. 14). 

 The principal constituent, quartz, predominates in the central 

 part of the knot, and is there coarser than nearer the periphery. 

 Just outside of the quartz core, which may also contain some 

 calcite, rutile, etc., is an annular belt of unevenly distributed 

 patches of calcite which encloses streaks and bunches of pow- 

 dery limonite. In one instance the shape of the belt was 

 rudely hexagonal. In another the limonite occurred in two 

 sets of parallel streaks which crossed the core. Such features 

 suggest pseudomorphism. 



The outer peripheral portion of the knot is composed of 

 finely granular quartz with chlorite and muscovite laths and 



