162 C. L. WHITTLE METAMORPHIC CONGLOMERATE. 



cite and quartz. These minerals may be distributed throughout the 

 feldspar, but there is a marked tendency for them to occur in groups 

 in the central part, and the outline of groups is rudely conformable to 

 that of the inclosing mineral. An explanation of this phenomenon has 

 never been offered that appears entirety satisfactory. It seems, however, 

 that we must be on the right track if we remember that the minerals, 

 quartz, sericite, biotite and plagioclase, are the commonly recognized 

 products of the decomposition of feldspar that has been subjected to the 

 influence of dynamic forces, while at the same time the presence of 

 quartz, sericite and albite making the groundmass of the rock would 

 be referred at once to such an origin by all who are familiar with the 

 ordinary microscopic phenomena observable in metamorphosed sedi- 

 •mentary rocks. Many feldspars (see those described below from East 

 Clarendon, Vermont) have interpositions of all other minerals occurring 

 in the rock, but the history of their immediate origin and growth is not 

 the same as for those under present consideration. 



Figure 5 is taken from the granulated portion of the above-mentioned 

 feldspar and exhibits diagrammatically an area under the microscope 

 covered by a number 5 objective : a is a portion of the normal microcline 

 pebble filled with inclusions ; a 1 , the same, with its interpositions removed 

 by the development of dynamic sericite, i i and k h ; a 2 , areas of elastic 

 microcline included in secondary plagioclase, c ; and b are minute prisms 

 of serecite grouped in the plagioclases, but impinging against the micro- 

 cline on one side in the upper area. 



At d are cleavage lines, emphasized by strain, showing abrupt disap- 

 pearance of inclusions along a line corresponding in direction to the basal 

 cleavage in the microcline h h and i i ; e, rhombs of limonite pseudo- 

 morphs after siderite; g, small liquid cavities arranged in lines parallel 

 to lines of hematite inclusions in the feldspar generally ; o, faint traces of 

 the double twinning of microcline occurring in the plagioclase. Two fad a 

 in particular are intended to be brought out : First, the grouping of seri- 

 cite prisms in the center of the lower plagioclase and their contact with 

 the clastic feldspar in the upper; second, the occurrence of isolated areas 

 of microcline in the plagioclase a 2 and the conclusions to be drawn there- 

 from. The plagioclase is known to be secondary, as it occupies areas in 

 and includes portions of granulated microcline once continuous. Sericite 

 is developed along the lines h h and i /, which are cleavage partings. 

 The linear area of microcline, a 2 , which is isolated, is optically oriented 

 with a 1 and has been separated from its parent along the basal cleavage. 

 Plagioclase surrounds and traverses it, dividing it into isolated areas, but 

 all oriented with one another. It seems to me that this is a case of pla- 

 gioclase forming about and from an area of microcline. The evidence in 

 favor of this is complete if all the data in the upper plagioclase are taken 



