OP ARTS AND SCIENCES. 277 



narrow bajids of a triclinic feldspar intertwinned with it on one side 

 (composition plane = plane of symmetry), while the broad binid of 

 supposed orthoclase is crossed here and there by twinned thin laminne 

 of triclinic ft-ldspar, probably having for the composition plane. 

 These two sets of triclinic individuals generally meet with a more or 

 less acute angle, but in one instance I obtained a very certain measure- 

 ment of 89° 30' — an observation that seems to leave little doubt that 

 we have here a combination of orthoclase with triclinic feldspar, similar, 

 as regards crystallographic structure, to the orthoclase-albite crystals of 

 Harzburg, described by Streng.* 



The thin sections of the rock we are describing leave no doubt that 

 ortiioclase is an essential primary constituent. 



I was able to measure some microscopic crystals directly under the 

 microscope, the angle 0: u getting 90° 10' as the result on an unstri- 

 ated individual, and 93° 29' on the striated ones. 



Measurements of the entering angle on the cleavage plane, with a 

 reflecting goniometer, gave 93° 24'. 



Optical measurements by Des Cloiseaux's method on sections parallel 

 0, and on random sections in the zone 0: ii, determine the feldspar to 

 be oligoclase. 



The most prominent appearance of change is the red staining, which 

 is due to the presence of countless minute particles of translucent red 

 or brown iron oxide, which is generally most abundant near the edges, 

 or along the cleavage planes, and in cracks. It is generally more or 

 less present throughout the crystals, though^ some of these are nearly 

 free from it, except along the above-defined lines. 



Where freshest, the feldspar is still clouded, and, under high powers, 

 this is seen to be due to the presence of innumerable minute particles 

 of irregular shape, of a bluish-gray color, and clear, except a narrow 

 dark rim, which seem to be inclosed particles of residuary base. 



Besides these, there are large numbers of partic^les of green sub- 

 stance in warped thick flakes or grains, which seem to aggregate to 

 form wisps or sheaves of long radiating needles. When examined with 

 only the polarizer, these last show an appreciable absorption. 



In many crystals, the mass is either clouded through and through 

 with the iron oxide, or is occupied to the extent of from twenty to fifty 

 per cent of its area by the sheaves of radiating green mineral, or both 

 substances are present. The green mineral also forms veins, filling 



* Streng. Feldspath Studien. Leonhard u. Geinitz, Neues Jahrbuch fiir 

 Mineralogie, 1871, p. 719. 



