90 THE CKYSTAL FALLS IROi^-BEARHSTG DISTRICl, 



In a more altered phase of the porphyrj- exhibited in a number of 

 specimens, the schistose structure is much better marked both macroscopic- 

 ally and microscopically. The macroscopical appearance is otherwise 

 quite similar to the one just described. Under the microscope the pheno- 

 crysts show up well. These are rounded and shattered orthoclase and 

 plagioclase feldspars. They lie usually with their long direction in the 

 lines of marked schistosity of the rock. The larger crystals have been 

 much more generally fractured than have the smaller ones, and seem to 

 have obtained relief from strain in that way, the indi^ddual fragments not 

 showing very strong dpiamic effects. The small crystals are more or less 

 rounded. The crushing to which the rock has been subjected has severed 

 the fragments in a number of cases. Triangular areas on two sides of the 

 broken or unbroken feldspars in the direction of schistosity are filled with 

 what appear to be secondary quartz and flakes of biotite. The feldspars 

 as a whole have undergone considerable chemical changes, the freshest 

 being red and very cloudy. Those more altered show secondary muscovite 

 and biotite scattered through them. The character of the triclinic feldspar 

 could not be determined. It appears, however, to be very rich in calcium, 

 as in some of the badly weathered sections the feldspar fragments may 

 almost be said to lie in a calcite matrix, resulting apparently from the 

 alteration of the feldspar and not from infiltration. No quartz phenocrysts 

 retaining their normal character are found. There occur here and there, 

 however, small rounded mosaics of quartz, the individual grains of which 

 show undulatory extinction. These are evidently the result of the granula- 

 tion of quartz grains, such as occur in the freshest specimens. It is well 

 known that the quartz is more easily affected by pressure than feldspar, 

 and Futterer^ has shown that they may be found in a completely crushed 

 condition, in the same section with feldspars which still retain their regular 

 crystal contours. 



The groundmass of the porphyry is made up of quartz and feldspar, 

 in and between which lie leaflets of biotite and sericite. The holocrystalline 

 granular mixture of quartz and feldspar is very fine grained, and the pres- 

 ence of the feldspar was only determined by difference in the refraction of 

 the two minerals. No striated feldspar grains were observed. The second- 



' Die "Ganggrauite" von Grosssachsen, und die Quartzporphyre von Thai im Thiiringorwald, 

 by Karl Futterer, Heidelberg, 1890, pp. 31, 126. 



