200 GREEN MOUNTAINS IN MASSACHUSETTS. 



The following is Mr. J. E. Wolff's summary of his notes on the Stone hill micro- 

 scopic sections: 



STONE HILL ROCKS. 



"We have in the quartzite series of Stone hill an interesting illustration of the 

 share that the original detritus and the modification produced by mechanical and 

 chemical agencies take in producing certain rocks. 



" The quartzite. varies microscopically from a finegrained rock, composed to the 

 eye of quartz grains and more or less mica to a coarse fragmental quartzite or fine- 

 grained conglomerate (locality 628) in which angular fragments of feldspar and rather 

 rounded masses or pebbles of blue quartz are visible; the latter grade insensibly into 

 the granular white quartz forming the rest of the rock. 



"Studied in the thin section the structure of the rocks is as follows: The large 

 masses of blue quartz show in polarized light that they have been subjected to great 

 pressure and strain, which-has resulted in a partial or total breaking up of the original 

 homogeneous quartz into a 'groundmass' or mosaic composed of extremely small 

 particles of quartz in which are contained cores of cracked and strained quartz which 

 are remnants of the original masses. 1 The comparatively large fragments of feld- 

 spar are seeu to be in most cases microcline or a plagioclase feldspar, but sometimes 

 without evidence of multiple twinning, and in that case probably orthoclase. The 

 substance of the feldspar is cloudy, owing to kaoliuization. The forms are sharply 

 angular and evidently detrital. The remainder of the rock is a very finegrained aggre- 

 gate of little grains of quartz and rarer oues of feldspar, the latter being similar in 

 character to the larger fragments of the same mineral. Irregular and interrupted 

 layers of a colorless muscovite, which has the wavy 'interwoven' structural form 

 characteristic of sericite, give the rock a lamination, the plane of which is parallel 

 to the planes of crushing in the quartz, that is, at right angles to the pressure. When 

 one of these layers of mica touches one of the large clastic feldspars, it often forks and 

 completely surrounds the feldspar, the two parts joining again on the other side; 

 accompanying this there is a thickening of the layer of mica around and near the feld- 

 spar, and sometimes litttle 1 ungues of the mica, branching from the main mass outside, 

 penetrate the feldspar, especially along cleavage cracks. It is therefore evident that 

 the clastic feldspar exercised an influence on the formation of the mica and probably 

 gave up part of its substance to form the latter. These large feldspars, like the 

 quartz, are fractured and broken, the quartz aggregate of the 'groundmass' tilling 

 the tissures. 



"The small feldspars of the 'groundmass' have in part the same characters as 

 the large detrital ones, and in fact are often evidently derived from an adjacent large 



'See PI. x in Tart n for an enlarged photograph of a thin section of this crushed blue quartz 

 from Stone hill. 



