360 F. H. Lahee — Metamorphism and Geological Structu 



re. 



Psammites. — The psammites are the commonest rocks in 

 the Basin formation. They occur at all horizons. As con- 

 trasted with the pelites, they are coarser, contain less carbon- 

 aceous matter, and number feldspar and muscovite among their 



Fig. 15. 



Fig. 16. 



Fig. 17. 



Figs. 15-17. Changes in biotite, which accompany increasing metamorph- 

 ism. Fig. 15 : low metamorphism ; the mica bears no relation to schis- 

 tosity, which is indicated in the dimensional parallelism of included quartz 

 grains. Fig. 16 : shreds along the edges of the mica bend into parallelism 

 with the schistosity. Fig. 17 : aggregates are elongate parallel to the schis- 

 tosity (arrow) ; the metamorphism is high. 



Fig. 18. 



Fig. 19. 



Fig. 20. 



Fig. 18. Garnet metacryst. The schistosity curves round it. Secondary 

 chlorite is developed at the poles of compression (right and left). The black 

 patch is ilmenite. 



Fig. 19. Hornblende metacryst, including quartz grains and an ilmenite 

 crystal. 



Fig. 20. Ottrelite metacryst, showing hour-glass structure (due to peculiar 

 distribution of included quartz and carbonaceous matter of the groundmass). 

 The black crvstals are ilmenite. 



original constituents. Those specimens which are least meta- 

 morphosed (Stage A) already exhibit alteration of orthoclase 

 to sericite, but this mica has no definite alignment. Both quartz 

 and feldspar grains still }:>ossess. their clastic forms (fig. 4). 

 Flattening of fossil casts and bending of the primary musco- 

 vite flakes are the only signs of distortion, and these are caused 

 by superincumbent weight and not by shearing. 



