PETKOGKAI'IIV— SAXIDIN THACHYTE. 497 



colored appearance as to the groundmass, with numerous, very lustrous and 

 glassy sanidin crystals and a few large grains of quartz, the two being 

 macroscopicall}' distinguished by the latter not showing any cleavage. 

 There is also some pinkish feldspar in the groundmass. In the section, were 

 observed large, clear sanidin crystals, some being cloudy and rather opaque, 

 and a few large and ver}- clear, rounded (juartz masses, all in a finel}' crys- 

 talline, feldspathic groundmass containing also numerous small quartz grains. 

 The large sanidins are very pellucid and generally show numerous fissures 

 and cleavage lines. They have also a dark border of alteration and fre- 

 quent cases of intrusion of the groundmass into the center of the crystal 

 In the last respect they are similar to those of [101] and [103]. The 

 other crystals are entirely cloudy or altered to almost opaque masses, 

 which, however, still preserve their outline distinct from the groundmass. 

 The quartz present, both large and small grains, polarizes in a brilliant blue 

 color. Some dark-brown particles are probably remains of decomposed 

 biotite. '^Phe groundmass is crystalline, being made up chiefl}' of small, 

 ragged feldspar crystals and opaque ones, so that with crossed nicols the 

 crystals are white on a dark ground and quite conspicuous, as are also the 

 blue quartz grains. The silica percentage is 72.27. There is some simi- 

 larity between this rock and [101], [103], [124] and [140]. 



The sanidin-trachyte [123] is from the same locality as the preceding, 

 but differs widely in appearance. It is bluish-green in color, somewhat 

 glassy and quite compact. White feldspar crystals are visible, and small 

 green needles in the groundmass can be seen with the loupe. In the sec- 

 tion, there are large and somewhat cloudy sanidin crystals, around which, 

 with a remarkable and beautiful fluid-like structure as if hardened at the 

 time of flowing, is a brightly polarizing groundmass, full of green horn- 

 blende crystals from large size to microlitic needles, the majority being of 

 medium size. The sanidin is generally in rather prismatic and rectangidar 

 crystals, quite dusty and opaque, or having occasionally a clear and polar- 

 izing central mass. There is a large crystal, which polarizes in very iri-egu- 

 lar blotches of color, with a very pellucid interior. In this point it bears 

 some resemblance to the large feldspars of [101] and [103], as also in the 

 abundance of hornblende prisms. At the angles and edges it appears 

 32 B u 



