28 GEOLOGY OF OLD HAMPSHIRE COUNTY. MASS. 



included minerals are in larger individuals, and so better titted for mineral- 

 og'ical study. 



THE BLUE-QUARTZ GNEISS. PERU.' 



This is a rustv, fine-grained gneiss, with little mica (biotite), in distant fiat 

 sheets of small scales, and with greasy blue quartz in grains and fiat plates 

 1 to 3"'" thick, which often coalesce into ])arallel layers of considerable 

 extent. These layers are plainly secondary infiltrations in a fine-granular 

 ground which has the aspect of a fine sandstone or cjuartzite. Under the 

 microscope this ground proves to be an exceedingly fine-gi-aiued mixture 

 of quartz, orthoclase, microline, and, in abundance, minute scales of nms- 

 covite, and it is such a structure as may have been produced by the crushing 

 of a granite and the change of most of its feldspar into muscovite. 



The lilue quartz contains a few minute bi'oken rutile needles, rarel}' 

 caAdties containing small, rapidly moving bubbles, and many sheets of very 

 fine pores or grains of some mineral. These are rudeh' parallel. There are 

 a few distant fissures. A fragment heated for a long time with the bellows 

 blowpipe retained its color without perceptible change. It .shows, with 

 plane-polarized light, small traces of undulatory polarization, and the 

 whole of each of the bands of the blue quartz, however large, polarizes as a 

 single individual. The sections were cut at right angles to the foliation, 

 but with what direction in that jilane I do not know. It is interesting that 

 in each case they are cut at right angdes to the optical axis, and the slide can 

 be moved from one end to another of the blue-(juartz bands — 1-2™"' wide, 

 15°"" long — and the optical figure remains sharph' defined, regular, and 

 unchanged, which would seem to militate against the explanation of the color 

 as due to strain. 



It is, however, a very remarkable fact that these slides still show the 

 lavender color distinctly with transmitted light when examined with the lens 

 or the eye alone, in spite of the fact that it is of so pale and dilute a charac- 

 ter that one would not expect to see it in so thin a film. Moreover, narrow 

 bands, at times branching, run across the colored layers, in which the color 

 is wholl}' wanting; and these bands, when examined in polarized light, are 

 made up of a fine mosaic of quartz fragments. It is thus plain that the 

 blue color is due to the state of tension in which the quartz is held, and 

 disappears when this tension is relieved by rupture across the mass. 



' Residence of H. A. Messenger. 



