

164 C. L. WHITTLE — METAMORPHIC CONGLOMERATE. 



the pebble, as though the plagioclase, as it replaced the microcline, took 

 on a crystallographic position, controlled by old planes in the latter that 

 had been emphasized by dynamic movements, in which one cleavage is 

 parallel to P or M in the microcline. Along these cleavage lines, at o, 

 there are faint indications of twinning, after the albite and pericline laws. 

 Plagioclase may have replaced old microcline twinning lamellae, itself 

 twinning after the same law, so that we may have the original twinning 

 of the microcline handed down to its alteration product, plagioclase. The 

 upper group of sericite inclusions, b, stops short in a plane against these 

 cleavage lines, extends across the plagioclase to the left, and impinges 

 against the microcline, which at this point is not crushed or strained, 

 while the feldspar on all other sides of the upper area is granulated. 

 Does not the area defined by these inclusions represent original micro- 

 cline, now replaced by plagioclase, the k going into the sericite? 



So complex have been the conditions of environment that it is very 

 difficult to interpret correctly, if at all, many phenomena exhibited in 

 metamorphosed rocks, but in this case it seems to be legitimate to sup- 

 pose that for a time the conditions were such that, as the plagioclase 

 formed from the microcline, sericite was developed. Afterwards the intro- 

 duction of new factors began. Sericite was no longer developed, while 

 plagioclase continued to form both from the microcline and by addition 

 from other parts of the rock, the growth taking place in all directions ex- 

 cepting where the area of inclusions abuts against the microcline which 

 had escaped granulation. Where this growth took place uniformly in 

 all directions the inclusions are seen to occupy a central group in the 

 plagioclase as in the lower area. The linear area of microcline, cr, with 

 its associated sericite prisms, is an intermediate stage between original 

 clastic feldspar and the completed change b. 

 ' In this connection Dr Wolff remarks : 



" It does not seem possible to explain all these cases of mere outward growth of 

 the feldspar grains by addition of fresh feldspar of the same species to the core, but 

 rather by an actual replacement of the detrital core by the feldspar of the enlarge- 

 ment."* 



Strongly contrasted as to immediate origin are the secondary feldspars 

 found in another phase of this conglomerate horizon and those occurring 

 in a phyllite in Massachusetts. 



CHLORITE SCHIST AND PHYLLITE. 



Their Plications built into secondary AlbiteS. — The differences in the envi- 

 ronment of growing secondary feldspars and their consequent histories 

 are seen when the above case is compared witli the porphyritic feldspars 



* Metamorphism of Clastic Feldspar in Conglomerate Schist: Bull. Mus. Com p. Zool., vol. xvi, 

 no. 10, p. 182. 



