ALTERATION OF THE FELDSPARS. 



163 



into consideration. The linear distribution of the microcline inclusion 

 and its optical relation with a are such that it manifestly was once a part 

 of the feldspar pebble. It is improbable that it could have been moved 

 in separate pieces from the line i i without having its crystallographic 

 and optic orientation disturbed. The twinning lamellae of the several 

 parts correspond with one another and with those in a\ while the ex- 

 tinction of c is not in parallelism with either set of twinned lamella? in 

 a 2 . The plagioclase c, resulting from the alteration of the microcline, 



Figure 5.— Thin Section of microcline Pebble. 

 Showing alteration of elastic microcline to plagioclase. 



ramifies through a 2 , apparently absorbing it, and at the same time caus- 

 ing the development of sericite about and in it. This will be alluded to 

 again when the areas of sericite b are considered. The arrangement of 

 the fluid cavities g 1 in lines parallel to the lines of hematite inclusions 

 in the microcline does not seem to be accidental, but seems rather to in- 

 dicate the operation of a grinding influence of the inclusions as the new 

 feldspar formed and absorbed the old. Cleavage lines d, which are par- 

 allel to i i, also seem to be connected in some way with the cleavage in 



XXIV— Bum. Ueol. Soc. Am., Vol. 4, 1892. 



